Why is it that kids tend to stop crying whenever you mock cry?

I don't know I always thought that was weird myself... but my son will stop crying if I do anything strange, I think its out of the ordinary for them so their wondering why your doing that, you've never done it before... I always thought it was strange how most babies cry when they hear another baby crying... I had to stop watching a baby story or any other show about babies on my TV for a while because if one would cry it would set my son off, its weird how things like that work.