(5 minutes plus time for glue to dry)
Glue the mirrors together, back to back. If you are using glass mirror tiles, tape the sharp edges - be careful! Broken glass is dangerous! Glue a wooden dowel to each mirror. The dowel should be positioned so that it sticks straight out of the middle of the mirror.
For a more durable assembly, use plastic mirrors and a dowel screw. This is a double-ended screw used to join two dowels. Drill a hole in the mirror and a hole in the end of each dowel. Insert the dowel in the mirror hole and screw on the dowels until tight.
(5 minutes or more)
Grab a dowel with each hand. While looking at one side of the mirror, move the hand on the other side of the mirror.
Your brain is fooled into thinking that the image it sees in the mirror is actually your other hand. When you move that hand, your brain naturally expects to see the hand move. After all, messages from the nerves in that hand tell your brain that the hand is moving. The hand's apparent failure to move is profoundly disturbing to your brain, which doesn't enjoy having its assumptions trifled with!
A simpler version of this experiment uses a single 12 x 12 inch (30 cm) mirror with no epoxied handles. Prop the mirror up on a table. Hold one of your arms on each side of the mirror so that you see the reflection of one arm as the continuation of the other arm. Snap the fingers on both your hands simultaneously, then stop snapping the fingers on only one hand. Or have someone drop an object (such as a set of keys) into the hand behind the mirror.