What do you have in common with a shark, a zebra, and a pine tree? All four of you are alive - and so all four of you are made up of cells. Cells perform amazing functions. An animal's muscle cells contract and expand in response to signals they receive from nerve cells, allowing the animal to run or jump. Plants' leaves contain cells that collect sunlight, which in turn provides energy for photosynthesis, the process by which plants create their...