You will need grapes (or any piece of fruit really) and a candy bar. This is a great object lesson to help the audience make the connection between obeying the Word of Wisdom and the need for proper nutrition.

Invite someone to come to the front of the classroom and instruct them to raise one hand and arm straight out from the shoulder. Place a piece of fruit in their hand with the instructions to try to keep their arm out straight as you apply pressure in order to push their arm down. Use two fingers to apply pressure to push their arm down. They will probably be able to keep their arm in place. Next replace the fruit with the candy bar and again follow the same procedure. While they are holding the candy bar it will be more difficult for them to keep their arm up.

This little experiment works with most foods. If something is held, that is more healthy for the body it is easier to hold your arm up. If you are holding something the body doesn't need, it is very difficult to keep your arm straight, even when the pressure is the same.

Choosing the right things to eat will strengthen us but if we use things that are not good for us, we become weak and have less strength.