There are a bunch of novelties that demonstrate physics. Some of these were on the shelf and some were in old encyclopedias and such. What would you show a kid to interest them?
I can think of more, but this is for everyone to fill out. Please don’t make a list of 20 experiments, and stick to only a couple.
I’m not sure if you’re looking for an off-the-shelf toy that demonstrates the principle, or just a simple experiment. If the latter…
Conservation of angular momentum - stand them on a small turntable with arms outstretched, then spin them slowly and tell them to draw their arms in to their body.
I’m surprised Mangetout didn’t have something; his site is full of neat stuff!
For me, I’d go with the Slinky Drop Experiment. No, I won’t tell you what it’s about, watch the video. At the end they ask you to guess the result before showing you what happens. I found it fascinating and very counter-intuitive!
The slinky has lots of neat things you can do with it; I can just imagine a group of kids, all with slinkys, all doing different experiments, and the chaos that would ensue! Such fun! Of course, knowing slinkys, and knowing kids, this could get expensive… you’d need a new batch of slinkys for each group of kids!
Only a few of my articles are explicitly scienc-ey, but OK - how about:
Pumpkin Seed Steam Jet Engines
I’m leaving it general so more people will participate.
You don’t give yourself enough credit! Lots of the stuff on your site has science behind it, if not as the specific point of the post at least the project deals with scientific ideas.
I’m still waiting for your new Stirling Engine, and your series of experiments with recycling plastic were pretty cool too (if perhaps not exactly child-safe.) The one where you weave plastic bags into a new bag would be good for kids, perhaps, but I can’t seem to locate that one.
Point is, anything is scientific if you want it to be!
My favorite is to make oobleck in a ziplock bag. (Corn starch and water mixed in the correct proportions to make a non-Newtonian fluid.)
If you hit it forcefully and fast, it’s hard and hurts your knuckles and won’t make much of a dent in the oobleck. If you slowly ease your fingertips into it and wait, it’s soft and squishy and you’ll sink right in as it moves around you. There’s a Life Metaphor in there, as well as a physics lesson.
I like the collapse a barrel. I saw Bill Nye do it at the local science center. A little water into a 50 gallon drum, heat it up, seal it, then roll it in a kiddie pool full of cold water.
The Crookes radiometer is what I can think of right now.
Actually, how about superconductivity? Plus you get to play with liquid nitrogen.
I recently hosted a science themed birthday party, and we had 5 types of experiments for the kids
Maybe something about Movable Pulleys?
I like energy type experiements such as catapults, trebuchets etc. Even jumping on an air bag and launching the person standing on it.
I don’t know how fancy you want to get, but I always thought that a cloud chamber was one of the coolest demos you could do. The problem is that the basic kits like the one in the link are very tricky to get to work - at least that was my experience.
United Nuclear used to have one that was more sophisticated but they don’t seem to carry it any more and I didn’t seen anything better doing a quick search.
Theodore Gray makes one in his book Mad Science, but says “it did take me several days and expert assistance before my home-made one worked”. Actually there are several experiments in that book one could try - electroplating, melting metal, making ice cream with liquid nitrogen, plasma torches, etc.
Nothing’s more fun than jolting kids with a couple hundred thousand volts.
Van De Graafs are fun.
The one that sticks with me from High School was not fully described to us, for fear we would go and replicate it. All we were told was it involved two substances, one containing oxygen, the other containing iron.
It was a few years before I heard it called thermite.
But it’s a very cool reaction. Smoke, flames and molten metal from some ordinary looking powders. And it has real world uses.
And yes, we would have replicated it… 
You require a high temperature burn to start the reaction or every kid would have used it.
SciAm has some good activities on Science Buddies. I just looked at the first page but these look good.
paper chromatography with flowers or candy
Also, this is again on the fancy side but worth mentioning, OpenPCR - a DNA amplifier for $600 (as opposed to $6000)
Took care of a burn once which involved an idiot putting thermite into a campfire and when it didn’t do anything, stirring it with a stick. :smack: That was a FANTASTIC burn which eventually, although thankfully slowly enough to treat with dressing changes instead of skin grafts, resulted in the degloving of his entire hand and the lower half of his arm.
Don’t try this at home kids, indeed.