Mass and gravity are difficult concepts to get your head around for the uninitiated. It took a tree falling on Newton's head to give him a nudge into starting the research, and we're still advancing theories today, but this is a cool one.
If you shrunk the Earth down in size, but not in mass - that is to say you made it physically smaller but maintained its weight - it would eventually become a black hole, so dense that the gravity it generated would suck in everything around it, even light. German scientist Karl Schwarzchild did the math for us shortly after World War I and calculated that for the Earth to reach this fearsome density, it would need to be 8.7 millimeters in radius. At that point it would essentially implode and drag in the rest of the Solar System; even the Sun.