Why do balloons deflate?

When I was younger I was into circusy things and received a balloon pump, several balloons and a balloon animal book as a gift so that I could practice my craft.  Recently I discovered the pump and some leftover old balloons and decided to try my hand at it again.  One thing that I quickly found out is that old balloons pop very easily when you're trying to inflate or bend them...

Of the few that I was able to successfully inflate and shape, I noticed that over the next few days the air would slowly leak out of them.  Obviously I had noticed this before, but I had never thought too hard about it.  I figured there are probably small holes that the air leaks out of, but I had never actually looked it up.

Upon closer inspection I learned that balloons are "not perfectly air-tight".  So the air molecules can slowly go through the surface.  However you might be thinking, "If they can go out why can't they go back in?  Shouldn't it be about equal?".  Well the pressure is slightly higher inside the balloon and so to equalize it the molecules are pushed outwards.

Apparently Mylar balloons have smaller "holes" than latex balloons, so they will leak slower.  Also helium balloons will leak faster than regular balloons because the helium does not interact very much with anything else and so will be smaller than the entire molecules of air.  Since they're smaller they'll have an easier time escaping.  Apparently you can also get your helium balloons coated so that they stay inflated for a longer amount of time!