What Is the Hubble Constant and Why Does It Matter?

Imagine you're blowing up a balloon. The more air you put in, the faster everything on it moves apart. The Hubble constant is like the speed at which the universe is expanding, how fast the balloon is inflating. Scientists use this number to figure out how old the universe is and where it might be heading. If they get the Hubble constant wrong, their whole story about the universe changes!