With the flick of a light switch, researchers can change the shape of a protein in the brain of a mouse, turning on the protein at the precise moment they want. This allows the scientists to observe the exact effect of the protein's activation. The new method relies on specially engineered amino acids -- the molecules that make up proteins -- and light from an LED. Now that it has been shown to work, the technique can be adapted to give researchers control of a wide variety of other proteins in the brain to study their functions.