Learners observe and compare the sizes of three toy “growing” crocodiles made from water-absorbent polymers. One is it its original state, dry, hard, and about 10cm long. One is immersed in salt water in a 2-liter bottle, a bit softer, and about 20cm long. The third is in pure water in a 2-liter bottle and completely fills the bottle. This is a long-term activity. Learners can observe and record the growth of the crocodile over a period of days; it can last in this state indefinitely.