A long time ago, in the land without ethics, before PETA....
Today in biology we learned that there is a gene in rats that can cause obesity if it's mutated. Scientists hypothesized that the gene created a transmiter in their blood stream that caused most cells to want more fuel, in essence, making the the rats hungry all the time. This hypothesis was tested by sewing a rat with the obesity genetic defect to a normal healthy rat, and connecting their circulatory systems. The experiment was a success, as trasmiters from the healthy rat flowing into the mutated rat's blood caused it to eat less, and reach a normal size. There's a similar gene in humans which can cause some people to be hereditarily obese, and can be cured with a pill. Isn't science fun?