1. Reverse the way a candy wrapper is wrapped on half of it so that you can't open it by just pulling on both sides.
Normally, one side is wrapped clockwise, and the other side is wrapped counter clockwise. Make it so that both sides are wound up clockwise.
2. Flip the tab on a soda can around so that no one will be able to open it.
See the pictures to see what I mean if you don't understand.