In order to understand black holes, it's necessary to be familiar with the concept of gravity. And one way to familiarize oneself with gravity is to explore space flight. In order to escape the enormous gravitational force of the Earth (relatively speaking - the force is strong for us who are stuck to its surface, but falls off inversely with the square of the distance, i.e. double the distance and you reduce the force of gravity by four) we need to supply rockets with enormous amounts of fuel. As always, Bobby asked the sharpest questions, including what would happen to the oceans if the Earth suddenly lost its gravity (not that that would happen, but Bobby's question on the hypothetical scenario was thoroughly amazing).