Catch or Pass

after Martin Gardner

Do you want to see if your friend can catch the postcard when you let it go? Just take a postcard in your hand and let your friend hold his/her fingers on each side of the postcard as shown in the illustration, but his/her fingers aren't touching the postcard. 

Let the postcard go. Your friend will not catch it. You can repeat it many times. The result will be the same - the postcard will easily "flow away" between your friend's fingers.