MAGNETIC NUCLEUS acts like a spinning top. The spin axis will normally align along the direction of a magnetic field applied constantly (center). A suitable oscillatory field can then induce the spin to reorient. For example, a 180-degree pulse (left) causes a spinning nucleus to flip entirely over. A 90-degree pulse (right) would force it to tip perpendicular to the constant magnetic field (vertical arrows). After it tips over, the spin axis will itself rotate slowly around, just as with a child's toy.