In this activity about physical stress (page 28 of PDF), learners discover that muscles and bones need to work to stay strong. In part 1, learners investigate how physical stress can affect bone (a hard material) by bending plastic knives. In part 2, learners explore the effects of stress on the muscles in their hands by predicting and counting the number of times they can "click" a clothespin. Learners will conduct this experiment every other day for two weeks to discover that their muscles grow stronger with conditioning. This guide includes background information, extensions, and a data sheet.