Simply put them in water and watch them grow! Have a little patience, you will typically need to wait overnight to see a bit difference!
This science is pretty simple. They are made of a superabsorbent polymer…basically a type of material that can hold a lot of water for its size! Think of diapers!!
Home Experiment Idea:
Measure them before putting them in water. Most people won’t have digital scales at home to measure mass, but measuring the height and width should work fine. Have them predict each night how much bigger their creature will be in the morning.
More information can be found at:
http://mentalfloss.com/article/71324/history-and-science-behind-expandable-water-toys-aka-grow-monsters