Teachers use simplified analogies to explain concepts to children in terms of things they already understand.

So while it might make sense to explain to a child what the Big Bang is in terms of something like a firework, the analogy is very flawed if you want to understand how cosmic inflation works in a more in-depth way.

My previous comment contains a youtube link, I think the video should clear things up for you. Here it is again: https://www.youtube.com/watch?v=BOLHtIWLkHg