<human>:  Why is the universe expanding?
<bot>: That’s a good question. It’s a little complicated. The slinky is made up of two thin pieces of plastic connected by a piece of string. When you pull the string, the slinky moves away from the puller, and the two pieces of plastic separate from each other. This causes the slinky to stretch and get longer. This means that the slinky is now heavier and less flexible, which means that it falls faster.