Explain · Like I'm five version
The Big Bang in terms of a popcorn machine
The Big Bang is just like a giant popcorn machine that started making the whole universe at once.
Everything in the world was once squashed together inside a tiny, hot dot, just like a single unpopped kernel sitting at the bottom of the machine. The universe was super small and very, very hot. Then, it suddenly decided to pop. It grew from a tiny speck into a massive space filled with stars and galaxies in a very short time.
Just like popcorn flying around inside the glass box, the pieces of the universe started zooming away from each other. They did not stop moving. The universe is still getting bigger today, just like the popcorn keeps filling up the machine until it is ready to spill out the top. The space between the stars is growing, which means the universe is stretching out like a big, fluffy blanket.
We can see the heat from that first big pop if we look very closely at the sky. It is like the warm glow left behind by the hot metal plate in the machine. Scientists call this the cosmic microwave background. It is a bit like finding a tiny, warm crumb at the bottom of a cinema bucket after the film has finished and everyone has gone home to bed.
Where the analogy breaks downThe universe does not have a glass box keeping it all inside.