Separate the stem from the flower and pull the stem into long strings.
The inside of the stem is hydrophilic, which is sometimes referred to as water-loving. It's the part of the plant that absorbs the water. And when it's placed into a tub of water, there's a whole lot of water to absorb! The water moves into the cells through the process of osmosis.
The outside of the stem is hydrophobic - it repels water.
The cells that make up the inside of the stem absorb so much water that they swell up. The cells on the outside of the stem stay the same size. The increasing size of the cells on the one side of the stem forces the stem into curls of various shapes.
There's definitely something fun about sitting outside on a warm day and watching the curls form! And if you can't be outside, grab some dandelions on your way to school and bring a bit of the outdoors in for your students.
PS The idea of one side expanding more than the other side is similar to the way a bimetallic strip in a thermostat works. The expansion is caused by temperature instead of water movement and it isn't as drastic as this, but it's conceptually similar.