Watch as your star slowly begins to expand and grow! What’s going on? This growing star trick works thanks to wood’s ability to absorb water. The water soaked into toothpicks and caused the wood to swell, just like a sponge expands when you soak it in water. The expanding wood caused the toothpicks to start to straighten. Because all of the toothpicks push against each other, they opened up to form the star shape you saw.
The water soaks into the wood because of capillary action: the ability of water to be pulled up into narrow spaces. Another easy way to see capillary action in action is to dip the corner of a paper towel in a cup of water. You’ll see the water move against gravity and travel up the paper towel. This is also how plants bring nutrients from their roots up to their leaves.