Show three wood blocks labeled A, B & C, along with a square tube. Block A is removed and put inside a hat. The remaining two blocks are covered with the tube. Ask the kids "Now where is Block A?" Of course they say "In the Hat." They are correct (this time)! You lift the tube showing the position of Blocks B & C. Block A is placed on top of B & C, then covered with the Tube. You ask "Where is Block B? Whatever they say is WRONG! You lift the Tube and Block B has VANISHED! Reach into your hat and remove Block B!
There are many routines and presentations that are possible with this wonderful magic show chestnut!