You take out a folded paper bag, unfold it and show it to be empty.
You then reach in and pull out a transparent plastic box with colorful flowers in it. The box measures around 6.25" x 3.75" x 3.75". You end up producing not one, not two but three such boxes!!!
When the three boxes are stacked up they stack up to be taller than the bag. It is impossible to have produced these boxes from the paper bag, yet you have produced it, right in front of the eyes of the audience.
This is an very well made prop that is very easy to do and yet is an very visual effect.