When I was a kid I loved doing simple science experiments.
One that I remember was putting an egg in vinegar and letting it sit. Eventually the vinegar will make the egg shell soft. (Although even now I’m not sure what science properties this exhibited…)
I also was fascinated with making crystals.
Share yours.
(Should this be in IMHO?)
Ah, man. My dad used to do cool experiments with me all the time when I was younger.
I remember once he showed me how you can separate water into hydrogen and oxygen with a DC power supply and a few jars. At the time, I thought that was one of the cooler ones. He also showed me how to make an electromagnet, which I then brought to class for show and tell to the amazement of all my friends.
And let’s not forget the time he actually bought me a robotic kit that we put together.
I can’t wait to share my love of science with my own kid!
The old baking soda & vinegar reaction is always fun, and can be used for various effects: diving buttons or raisins, inflating a balloon, volcano (see also Mentos and Diet Coke).
Heat up a little bit of water to a boil in an aluminum soda can, invert can into some ice water and WHAM! the can is crushed by atmospheric pressure when the air in the can rapidly cools. Can also be done by boiling a little water in a metal gas/kerosene can, screwing the lid on, and letting it cool gradually. But the ice-water cooling is more dramatic!
One summer I tried to conduct an experiment on what foods ants preferred. It didn’t turn out so well.
Not one to do at home, but the most memorable one we did at school was to add concentrated sulphuric acid to sugar, forming a big carbon lump: link
I used to do that! Good thing my parents didn’t find out. 
Another thing that was fun was playing with the empty CO[sub]2[/sub] cartridges from my BB pistol. I’d use a syringe to fill one about 1/3 full of water, and then plug the hole with a toothpick. Then there would be an application of heat (Sterno can) while I held the cartridge with kitchen tongs. When a little steam was pushing its way briskly past the slightly-loose toothpick I’d pull the toothpick out with a six-inch tweezers and let go of the cartridge. It would go flying! My dad showed me this one. Only he did it in the kitchen!
Lemon-powered light and motors. Good times.
Speaking of lemons…
Invisible ink. Dad and I were camping on my 8th birthday (I think it was). He used lemon juice to write a clue on a piece of paper, and we heated it over the Coleman lantern. I followed the clue and found… another clue. We heated it up with dad’s Zippo lighter. Clue after clue eventually led to my birthday present. (It was a battery-powered boat called ‘Renegade’. 
)
I remember one time my buddy and I tried to make nitroglycerin. One experiment, we made what we thought was the real thing. (No, no nefarious purposes)
Get this: we put a couple drops of what we thought was the real thing on a vice, and hit it with a hammer to see if it would explode! (Nothing.)
Another experiment: we got certain chemicals together at the right time and the right temperature (no, I don’t remember what), looked again at the chemistry book and read that if the ingredients started giving off smoke it was about to explode! Well, we hauled ass out of the basement expecting his house to be totally vaporized. Lucky for us (and the neighborhood)this didn’t happen. We went on to making stink bombs after that. 
This is back in the 1950’s for and to whom it may concern.
ETA we were in High School when this was going on.
I always liked coloring flowers with food coloring. I could make some wild looking bouquets with the flowers out of grandma’s yard.
My son enjoyed the one he called “psyching out the bean sprout”. We planted a bean in a small container and put it in the bottom of a shoebox standing on its end. Then we used pieces of cardboard to create a simple maze, cut a hole in the top end of the shoebox for light to come through, and put the lid on. In a couple of weeks the bean sprout had wound its way through the maze and out the hole. Son won a blue ribbon at the elementary school science fair for that one.
My favorite one has a scientific name that I don’t remember (something something solid).
You mix corn starch and water. If you get the proprortions right, you get this … stuff. It looks like a liquid when it’s just laying there, but as soon as apply pressure to it (by picking some up in your hand) it turns into a solid. Then when you drop it it turns into a liquid again. It’s lots of fun to play with.
I think the term you’re looking for is non-Newtonian fluid.
I know young kids have been fascinated by chromatography; grab a coffee filter and use a flat area of it to make a test strip. At the bottom of the strip, make a line of dots from different markers of different colours and brands, and dip the end of the strip in water. As the water gets pulled up through the filter, the dyes in the dots will come apart and you can compare them to each other. Seeing which colours are in the black marker seems to be a favourite with kids. I’ve never bothered to try using other solutions as the mobile phase, though I’m sure using vodka or rubbing alcohol would result in a different separation pattern, since that is pretty much the entire theory behind chromatography in the first place!
In addition to non-Newtonian fluids, kids love the slightly less messy “silly putty” made with water, Elmer’s Glue and Borax. I made this for students in my mom’s class last year (actually three grade one groups and a grade 6 class that happened to be studying polymers) and it was a hit with everyone.
Dry ice in eppendorf tubes were always entertaining in my chem labs in university, especially because some could take a while to pop, so you couldn’t know who did it! Freezing and smashing fruit in liquid nitrogen is also bizarrely fun.
Down the supermarket you can buy these volatile ingredients:
Make sure you have a large open area and safety goggles when you try this!
Yep, that’s it, Thanks! Fun stuff.
I also used to do pixie sticks dumped in soda. Pretty much the same effect as mentos and diet coke, but not nearly as spectacular.
Just a couple months ago I dropped sand into a spice bottle full of Mountain Dew to see what would happen after a couple weeks. The answer: nothing. But it did provide mega-nucleation sites so it fizzed all to heck when I dropped the sand in, and what’s even cooler, this nucleation even took place as the sand was settling in place, so there were lots of decent-sized bubbles “frozen” in the sand for a couple days!
Never did this one, but this video of Hot Ice sure looks cool. Youtube
In the video they dissolve some sodium acetate in near boiling water, then cool it down to around room temperature. When someone touches it with their finger it sends out spider-like crystal throughout the whole “Ice Cube”. (57 seconds in).
And here’s another one I’ll link to because someone mentioned it in GQ the other day. Egg sucked into soda bottle.