Toothpicks are made of dry wood. When I broke them in the middle, the wood fragments inside compress. Once we add water to the center circle of the star, capillary action causes the water to be absorbed into the toothpicks.
The water moves inside the dry toothpick from the starting point of the crack and continues along the length to the pointed tips. The capillary action or water traveling inside the toothpick causes the toothpicks to straighten out. The set up with 10 points creates a 5 point star.