Get a plastic bottle and a balloon.  Make a hole in the bottle with something – it doesn’t have to be a sharp implement.  For instance, a pen should do.

Push the balloon inside the bottle, and stretch the neck of the balloon over the mouth of the bottle.

Now blow into it.  The balloon inflates.  (Of course it does.)

While you’re still blowing, put your finger over the hole in the bottle.  Stop blowing, and take your mouth away.  The balloon stays inflated!

When you blew up the balloon, it pushed air out of the bottle.  When you put your finger over the hole, it stopped any air coming back in.  The air left inside the bottle has too low a pressure to deflate the balloon – and so it stays inflated.