This one occurred to me as I was lying awake this morning.
You know how you learn a new language - from someone who knows both your language and the language you want to learn?
Well, how did the first person to learn a different language learn it?
Sure you can hold up a rock, point to it and say "Rock" while the other fella says "Ugga" or whatever but how do you cover verbs, tenses and all the technical stuff?
You could probably demonstrate running but how do you cover ran?