Slow Children Ahead

That has always been one of my favorite road signs. How did they know retards would live there?
It brings up an interesting point, though. Like, how do parents talk to kids that...well...to be more polite than my first line...are not the brick with enough mortar?
Normal conversation between parent and reasonably intelligent child going away to college: "Don't forget to write!"
Conversation between parent and child with low-watt bulb for brain: "Don't forget how to write!"