You have to speak a bit of German.
Fear is how you pronounce the German word for four (vier) and sex is a bad pronunciation of the German word for six (sechs).
The German word for 5 is funf, the word that comes between 4 and 6.
Of course, by the time you explain this, people have stopped rolling the aisles from laughter.
Am I just dumb? I don’t get these at all.
I was going to tell this duck joke I liked and of course by actually googling it I found it online, so I’ll just cut/paste:
A duck walks into a convenience store and asks the clerk, “Got any grapes?”
The clerk says no, and the duck leaves.
The next day, the duck returns and asks, “Got any grapes?”
The clerk again says no, and the duck leaves.
The day after that, the duck walks in the store again and asks “Got any grapes?”
The clerk screams at the duck, “You’ve come in here the past two days and asked if we had any grapes. I told you no every time that we don’t have any grapes! I swear if you come back in here again, and ask for grapes, I’ll nail your webbed feet to the floor!!”
The duck left, and returned the next day. This time he asked, “Got any nails?”
The clerk replied, “No,”
and the duck said, “Good! Got any grapes?”
Here’s one that usually gets a laugh when I tell it:
Gary Smith was riding the crosstown bus when a passenger boarded and took the seat next to him. They struck up a conversation, during which Gary learned that his seatmate was a licensed sex counselor. He felt a bit awkward, but he finally blurted out that he and his wife, though they had a fairly good sex life, were having one minor difficulty. “You see, Doc, we’ve never been able to actually finish together–I’m never able to wait for her. I always make sure that she’s satisfied, but still…”
“You know, Gary, it’s funny you should bring that up, because my wife and I used to have the same problem. But we figured out a way to fix it and we’ve been happy as clams ever since!” replied the counselor, “This is how it works–before we begin, I place a .45 loaded with blanks under our pillows. Then when I know I’m about to finish too soon, I whip out the gun and fire into the air. The sudden noise distracts me slightly, but it really excites my wife, so I slow down and she speeds up. It’s just enough to keep us in sync.”
“Hmm… that’s pretty unusual, but it sure sounds worth a try,” Gary said. “I’ll go get the gun and blanks right away!” "Here’s my card, " said the Doc, “Call me and let me know how things went!” With a wink and a smile he got off the bus.
About a week later, the expected call came. “Well, Doc, I took your suggestion,” said Gary. “I decided to surprise my wife and slipped the loaded gun under the pillow without telling her–she’s always saying I’m not spontaneous enough. So we were having a great time and suddenly I knew it was going to end way too soon. Just in time, I remembered to grab the gun and I fired a shot into the air.”
Here Gary paused. “Well, what happened next,” prompted the doc.
“She shat in my face and bit off my dick!”
Knock, knock.
Who’s there?
The Prophet Mohammed.
The Prophet Mohammed, who?
DIE, INFIDEL!!!
I don’t get it…
Clayton, a good ol’ boy, is involved in a minor fender-bender. He gets out his truck and walks to the driver’s door of the shiny new Lincoln he just bumped into. The driver of the Lincoln gets out in a stiff gray suit and says, “Gentleman, let us assess the damages.”
Clayton says, “Sheeet, doc, I was mindin’ my own and ain’t see no brake lights.”
The Lincoln driver says, “I am not a doctor, but I am a Professor. I offer you my card and an opportunity. Disregard the damages and consult me when you feel it will be most beneficial to avail yourself of my services.”
Clayton takes the card and responds, “Sir, I don’t know what you talkin’ bout, but I 'ppreciate it. I’ll be on my way.”
Two weeks pass and Clayton calls the number on the card. After introductions are made, Professor Ramore asks Clayton if is interested in attending college classes. Scheduling and financing would be taken care of by the Professor.
Clayton asks what classes he would have to take.
“Well you would have to take mathematics.”
“That’s numberin’ I’se good with the numbering”
“You would have to take composition.”
“That’s a big word for writin’. I can write. I rote my name in pen once.”
“You would also have to take logic.”
At this point, Clayton faltered. “What the hell is logic?”
The Professor asked, “Do you own a lawnmower?”
“Well yeah, a Briggs and Stratton, but what that got to di with anything?”
“Well,” the Professor responded, "if you own a lawn mower, I would logically presume you own a home. If you own a home, I would logically presume you have children. If you have children, I would logically presume you are a heterosexual.
Clayton says, “Damn, doc, sign me up for those classes. I like the way you think.”
Two week pass and Jethro comes over to visit Clayton.
“Hey, Clayton, I ain’t seen you in days. Where you been?”
“Jethro, I’m getting me an education.”
“Really, what they teach you?”
“Well, I learned me some mathematics.”
Jethro says, “That’s just numberin’ I can do that.”
“I learned me some compo-composit–composition”
Jethro leans in and says, “That’s just a big word for ritin’ What you really learn?”
“Well,” says Clayton, “I learned me some logic.”
“Really, what’s that?”
Clayton asks, “Do you own a lawnmower?”
Jethro says, “You know I don’t.”
Clayton responds, “Faggot”
SSG Schwartz
Could someone explain?
The first orgasm is easier to reach than each following one.
A church’s bell ringer passed away. So they posted the position and a man came in with no arms wanting the job. The clergy weren’t sure he could do it, but he convinced them to let him try it.
They climbed the bell tower and the guy ran toward the bell and hit it with his head. They gave him the job.
The next day he went to ring the bell, tripped, bounced off the bell and fell to the sidewalk below. Two guys were walking past.
One asked, “Do you know this guy?”
The second guy responded, “No, but his face rings a bell.”
The next day, the dead bell ringer’s twin brother comes in for the again vacant bell ringer position. He also has no arms. They lead him up to the bell tower, he runs at the bell, trips and falls to the sidewalk below.
The same two guys walk by.
The first asks, “Do you know him?”
The second guy responds, “No, but he’s a dead ringer for the guy we saw yesterday.”
Sure, but wont she still be charging for whatever time he spends masturbating in front of her?
That is why he is masturbating furiously, when she comes out. IOW woosh.
SSG Schwartz
I tell that one, and then, before the laughter subsides too much, I tell this one:
Knock knock
Who’s there?
An interrupting clam
Interrupti…
[immediately bite both your lips, stretch your cheeks back, open your eyes as wide as you can, and thrust your face at your victim, chasing him a bit when he recoils in terror]
Knock, knock.
Who’s there?
Eskimo Christians.
Eskimo Christians who?
Eskimo Christians, I’ll tell you no lies.
This one came first in an internet survey to find the world’s funniest joke.
Two hunters are out in the woods when one of them collapses. He doesn’t seem to be breathing and his eyes are glazed. The other guy whips out his phone and calls the emergency services. He gasps, “My friend is dead! What can I do?”. The operator says “Calm down. I can help. First, let’s make sure he’s dead.” There is a silence, then a shot is heard. Back on the phone, the guy says “OK, now what?”
And then there’s this from the immortal Jackie Mason.
My grandfather always said, “Don’t watch your money; watch your health.” So one day while I was watching my health, someone stole my money. It was my grandfather.
Sorry if I offended anyone with that science fiction/Muslims joke.
There are some great tasteless jokes here: http://boards.straightdope.com/sdmb/showthread.php?t=436935&page=1
This is also worth a look: World's funniest joke - Cafe Society - Straight Dope Message Board
Rome, cobblestones, etc. 
How many perverts does it take to put in a light bulb?
Just one.
But it takes the entire emergency room to get it out again.
A variation on the two hunters joke, sort of:
Bill and Bob were hunting and Bob had to pee, so he went behind some scrub and started to pee. Unfortunately, he surprised a rattler, which struck and bit him on the penis.
He told Bill, who had a cell phone, to call the doctor and find out what to do.
Bill called the Dr. and asked him what to do.
The Dr. said “you need to suck the poison out of the wound”.
“Really?” asked Bill.
“It’s the only way to save him” the Dr. replied.
Bill hung up.
Bob asked “Well, what did the Dr. say?”
“You’re gonna die” Bill replied.
Q. How can you get two elephants in a mini ?
A. One in the front, one in the back.
Q. How can you get two hippos in a mini ?
A. None, the elephants fill it up.
Q. Empty the car. Now how can you get two whales in a mini ?
A. Down the M4. 
Assume you’re in London.
How can you get to Wales in a mini? Down the M4 (motorway)
I actually do know a little German and I stared at the punch line for 10 seconds before I realized it was supposed to be fünf. Granted, I was still giggling from one of the earlier ones, but I would strongly suggest to anyone trying to get a laugh with that joke that they at least pronounce the word correctly.