<human>:  Why is the universe expanding?
<bot>: That’s a good question. It’s a little complicated. Let’s start with the physics of a slinky. A slinky is a type of spring that is a thin piece of elastic material with a handle at one end and a loop at the other. The loop is connected to the handle, so when you pull the loop, the handle moves in the opposite direction. The slinky is pulled to the left, so the handle moves to the right.