First, you'll need to find the "equipment". You're looking for some sort of 'toy' that has two contact points, like those shown above. The toy will 'do' someting when the circuit is completed: mine are "ice" cubes that light up in water, I've seen Easter chicks that cheep when the circuit is complete. Unfortunately, I can't tell you exactly where to find these, you just have to keep your eyes open. My best advice is to check out Walgreens/CVS/RiteAid around the holidays.
Once you have procured your supplies, the activity is very simple:
Have the class stand in a large circle. Everyone needs to hold hands. You'll be a part of the circle, one of your hands will hold a student's hand. In the other hand, you'll hold the 'toy', with one of your fingers covering one of the contact points. You'll need a student to stand on the other side of the 'toy', covering the other contact point.
If the entire class is holding hands, you'll have a complete circuit and the "ice" cubes will light up, the chick will cheep, etc. If there is a pair that isn't making contact, nothing will happen.
Expand the activity by looking for some conductors and insulators to include in your circuit!
In addition to a discussion of complete vs. incomplete circuits, it's also a chance to talk about electrolytes in your body (hence the ability for electricity to travel through bodies).
