Before strenuous physical activitiy it is very important to warm up the muscles and tendons. Cold muscles have less flexability and cold tendons have a much higher incidence of tearing and producing injury. Below is a simple and effective way to demonstrate this.
Materials: A "frozen" stick of sugarless gum for each student - long thin sticks will work best.
Procedure: Before any physical activity begins, have the students sit on the floor in a butter fly stretch (holding feet with hands in close to body, elbows on knees). Gently stretch the legs and back, then extend legs straight while reaching for them to stretch the hamstrings - do not exceed 30 seconds per stretch. Have them note their current "cold" flexability and how it feels (tight? sore?).
Now pass out the frozen gum sticks. Have them unwrap and bend the cold stick in half. If done in time, the stick of gum should snap in half. Let them know this is similar to putting strain on a cold muscle.
Now have them place the 2 smaller pieces backinto the wrapper and into 1 palm, and close their hand around the gum while they perform the next light warm up excerces. Instruct them to do light movement like jogging in place, dancing, hopping, jump rope, etc). The light activity gets the muscles warm. Perform these activities for 5 minutes, then have them sit down again.
While sitting have them bend the gum pieces in half again. they should fold over easily without breaking. This is representative of how much more pliable warm muscles are. Now they can chew the gum. Have them perform the stretches again now that their muscles are warm. Hopefully they will also find this easier then when they were cold. Be sure to tie in how important it is to warm up before physical activities to lessen the chance for "breaking" and injuring muscles and tendons.