There are lots of reasons not to stick chewing gum behind your ear—such as, why would you want to in the first place?—but the best one of all is painfully obvious: don't put chewing gum behind your ear, because it will get caught in your hair, which, if you don't own scissors, you will then have to cut with a pair of pruning shears, and while the long handle might provide plenty of leverage, the distance between the blades was designed with arborvitae and not human hair in mind, meaning that you may have to use a forward and back motion with one isolated blade, and by that point, you may as well have used a knife.