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From Fly Like A Butterfly: Yoga for Children, by Shakta Khalsa:
During the relaxation phase of children’s yoga you can do the Spaghetti Test. You will alternate a few times between creating tension, followed by relaxation of the entire body, including the face.
Here’s how it’s done:
While lying on their backs, have them stiffen every part of their bodies, and be like spaghetti before it is cooked. Slowly count to 3. Then ask them to completely relax while you slowly count to 3. Tell them to be like cooked spaghetti. Then go around and test them to see if their arms are relaxed. With respect, go to each quietly and lift the arm and jiggle it a bit. You can test the legs too (make sure to support the knee). I like to tell them, “We have so much cooked spaghetti that we are going to have to make a spaghetti dinner!” 
Once they learn how, they can try it on each other as well. It is great biofeedback; they will really begin to feel what relaxed and tense feels like!
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