As amazing as it may sound, "Do Not Spit on Floor" signs used to be a prominent feature in public buildings. If you were to see a sign with such a message today you would probably stop to laugh at it, but the signs were made for a reason. People used to spit on floors.

Things such as not spitting on the floor may seem common sense to us now, but at one time evidently they did not. It takes time for people to learn new things. Nowadays we see signs that say "Do Not Litter". Think how absurd they might seem to a person twenty or thirty years down the road if we eventually manage to get our act together on the environment. Education is the key. People can learn anything over time.

If a sign used today is seen as silly in the future, we know it has done its job well.