The bomber will always get through

Okay kiddo, imagine you are playing a game of catch with your friend. You are the catcher and your friend is the thrower. Now, imagine your friend has a really strong arm and can throw the ball really far. Sometimes when they throw the ball, you just can't catch it. It's too fast or bounces in a weird direction and you miss it.

Now imagine that instead of playing catch, your friend is trying to throw a bomb at a target. There are people whose job it is to stop your friend from throwing the bomb, but sometimes they can't do it. Maybe the bomb is too powerful or the person throwing it is too sneaky or clever.

So, even though we try really hard to stop bad guys from doing bad things, sometimes they are just too good at it and they succeed. That's what people mean when they say "the bomber will always get through." It doesn't mean we should stop trying to prevent them, but we have to be aware that sometimes bad things can still happen.