Set up two chairs back-to-back with about 2' - 4' of space in between them.
During this activity, the children will take turns standing between the chairs with their arms extended slightly to the sides (i.e. approximately 10" from their waists). The backs of their hands will touch the backs of the chairs; there should be just enough space in between the chairs so that the children can do this.
Have a guest sit in each chair to secure it, and then have the other children take turns standing between the chairs with their palms facing inward toward their bodies.
Tell them to push their arms outward against the chairs, keeping both arms straight as you count to 30.
Once you've finished counting, have the child whose turn it is step forward, away from the chairs, and relax his/her arms.
The child's arms should raise up very slowly on their own, without any effort by the child. The children will be completely amazed at this phenomenon!
You and your older guests can try this exercise, too; just stand inside of a doorway rather than using chairs.