In this activity (on pages 25-31 of PDF), learners soak sponges with different amounts of plaster of paris to simulate different levels of calcification in bone formation. After being allowed to harden for 48 hours, the different simulated bones are tested by loading them with weight until they break. The lesson plan connects this to calcium nutrition and osteoporosis, and contains advice for building up calcium in bones. The activity is suggested for grades 5-8, but can be done as a demonstration for grades 3-4.