One of the experiments we've come across multiple times, in countless experiment books for children, is the underwater volcano.
Start with a tall glass jar, full of cold water (we used an old fish bowl).
Then, place drops of food coloring, and hot water, into a small necked bottle, or jar, that is shorter than your first jar (we used an empty almond extract bottle).
Most books suggest a complicated procedure of looping a string around the neck of the small bottle, to make handles, for lowering it into the large jar. We thought it was easier to just hold the bottle with two fingers, while covering the opening with a thumb, and then placing it on the bottom of the fish bowl, by hand.
Once, the opening is uncovered...
...the "eruption" begins.
It's a lot of fun to watch.
And, it works for the same reason the smoke, from our recent forest fire , "erupted" so high into the air...
...and for the same reason we were able to fly our solar balloon.
Heat (from the fire, the sun, or the heating elements in the hot water heater), speeds up the molecules in the water (or air). The faster moving molecules make the water (or air) less dense, than the colder water (or air), with it's slower moving molecules. So, the less dense water (or air) rises.
We thought the best part of the underwater volcano, was watching the cooling drops of water start to fall - the children thought it gave it that hurling-lava-rock type quality, necessary for a really good volcano.
It's great to be a homeschooler.