You guys should build a Tesla coil!
Oh, why couldn’t you be MY uncle? :dubious:
How about how yeast ( a living organism) makes bread rise. You could buy a packet or three of yeast and also use a beer bread recipe ( to compare the two) Then you can EAT your project.
Try that with lye.
How about a Time Travel Machine?
I don’t know enough about electronics to build one. I’d LOVE to get one of those 60 projects in one kits at Radio Shack and my niece and I could learn about electricity together.
I used to love this one as a kid:
(This takes a little skill)
Take a small piece of bristol board (like a playing card) and cut it into a stylized boat shape (make the boat less than an inch long). Cut a notch in the back. Take a bar of soap, and slice off slivers of soap using a knife. Take an appropriate sized sliver, and wedge it in the notch, like an outboard motor. Carefully set the boat in a bowl of still water, and watch it zip around!
Why?
Surface tension - the soap reduces surface tension behind the boat, which pulls the boat forward.
Take two sheets of paper, and hold them vertically in front of your mouth. Ask the kid to predict what will happen when you blow between them.
Kid will say “they will be blown farther apart!”
Blow.
Then let the kid try. She’ll eventually figure out that the harder she blows, the more the sheets get pushed together. Bernoulli’s Principle in action.
You could show the different densities of gas by making carbon dioxide with vinegar and baking soda. You can test it by pouring the gas on a tea candle in a cup which will extinquish the flame.
Or you could show the expansion of gases by heating a teaspoon of water in a 2 liter bottle (bolling it in a pot of water). When the water is turned to vapor you put the cap on it and then run it under cold water to see the bottle collapse when the gas cools. This is really impressive if done with a 55 gallon steel drum over a fire. it completelly crushes the drum.
Well, THEY’RE no fun. No fun at all. Have you checked out American Science and Surplus? That’s a link that goes to their science project books, and I’m sure you can find at least one that will keep you and your niece busy for several months.
Some very simple electric motors:
Pay attention when it tells you how to partially strip the wires.
and a very simple electric generator:
http://www.eskimo.com/~billb/amateur/coilgen.html
My kids were too old to care much when I got interested (I hate to use the word ‘obsessed’ when talking about myself) in these.
However, everybody else is pretty comfortable with it. 
Try the exploding grain elevator simulation. Grain elevators can be dangerous because they fill with dust from the grain, so there are tiny aerosol particles of fuel (grain dust) surrounded by air, so if there’s a spark, each little dust particle ignites with the adjacent air, lighting the next dust particle, etc. - causing the grain elevator to explode. In essence it is an air/fuel explosion, not unlike when gasoline vapor gets lit and explodes.
Attacklad (6 or 7 at the time) and I simulated this one day when Ms. Attack was out. We used a cardboard bulk raisin can (like a shorter, wider Pringles can). We cut a small hole at the base of the wall of the can, just big enough to admit a drinking straw. We put a little pile of flour on the end of the straw in the can, so that blowing sharply in the straw would propel a cloud of flour up into the can. Then we stood a lit zippo lighter on the floor of the can. We used a zippo because it would burn reliably for a while.
With this setup, if you blow sharply in the straw, the flour flies up in a cloud, hits the zippo flame, ignites and explodes upward. We were getting bursts of flame 3-4 inches above the rim of the can. Tons of fun.
Now that I’ve thought about this again, I’m wondering - how well would powdered sugar work?
Might be a good idea to put down some newspaper in case you get flour everyplace.
PS Actually T. Slothrop’s electromagnetic widgets are way fun. I’ve got to get my supplies out again now that my kids are older.
The demonstration of brownian motion is always interesting. You’ll need a microscope of around 50x - 100x and a chamber to trap smoke (can be improvised with a couple of microscope slides).
I remember a science experiment book describing how to simulate grain elevator explosions - they said to use “Lycopodium powder.” I never could find a source as a kid…
Lighting things on fire with a magnifying glass is popular - perhaps too popular - we had to hide the magnifying glasses.
This “robot bug” was popular - we did it a little different than the instructions, but you get the idea… http://www.scienceworkshops.org/files/_shared/projects/Minibot.doc
You could bring two ice cubes out of the freezer, and pour some salt on one. There’s some complicated science there, but the crux of it is that a solution generally has a lower melting point than pure water. Therefore, the ice/water mixture will be liquid even when the substance is around -10C, instead of 0. It doesn’t actually change the temperature of the mixture, but allows it to shed its latent heat of fusion more quickly.
I’ve never tried powdered sugar, but non-dairy coffee creamer works quite well. It doesn’t even need to be confined. And, I hate to be pedantic, but it’s a deflagration, not an explosion.
ETA: I know it’s often described as an explosion, and you get 100 times more hits by googling grain elevator explosion compared to grain elevator deflagration, but it’s still not an explosion; without much oxygen and nitrogen (relatively) in the organic molecules, you’re not going to be able sustain a combustion propagating faster than the bulk speed of sound in the material.
I protest!
The point of doing a science project isn’t to do a test, it’s to figure out a question and a way to determine an answer to that question. If you get both of those from a book, you’ve already failed the project for all intents and purposes.
Sure, the kid with the baking soda volcano will get all the cheers, but he won’t have accomplished anything educational.
You should tell him to go outside and watch everything he sees and anytime he sees something that makes him wonder, “Huh, I wonder why X is like Y?” or “…what causes X?” or “…why X does Y?” Then choose whichever one seems like there might be some strategy for solving it.
It is an explosion: Explosion - definition of explosion by The Free Dictionary
although those of you in the biz may prefer a narrower definition. I’d like to see you argue that confining dry ice in a frangible container is not an “explosive” to a judge.
Some assembly required, but how about a water rocket?
You can try experimenting with different water levels in the bottle to see what gets it to go the highest.
I understand your objection. But I think my niece is a bit too young for this. Also, simple demonstrations can be very educational. She’s already learned that all magnets have two poles, that electricity must flow in a circle and much more.
Oooh. This one. This is the demo answer to “How do you pick up an ice cube with a piece of string?” 
There’s always dry ice bombs!