In some atoms, the most likely way for an electron in the n=3 state to reach the ground state is to decay first to the n=2 state and then to the ground state. Making the transition from n=1 to n=3 requires a photon of a particular energy - the photons emitted in the n=3 to n=2 and n=2 to n=1 transitions have lower energy.
By absorbing ultraviolet light electrons can be excited from the ground state to the n=3 level. Visible light of a particular color is emitted when the electron drops from there to the n=2 level.
If the transition from n=3 to n=2 takes place quickly we call this fluoresence. In some materials the electrons take a long time to decay from the n=3 state so the visible light they emit lasts a long time. That is know as phosphoresence.
Fluorescent lights are practical examples of this. Electrons that have been accelerated by a high voltage collide with mercury atoms in the tube, exciting them. These atoms emit ultraviolet light when they decay back to the ground state. The UV hits the fluorescent coating on the wall of the tube, causing visible light to be emitted.
We can "activate" our yo-yo by placing it on the overhead projector for a while. Activating it means that we are exciting electrons to a higher level. When we then turn off all the lights we can still see the yo-yo glowing. This tells us that the yo-yo is:
The yo-yo is phosphorescent, because we can still see the light long after the source that activated the yo-yo is removed. This is in contrast to our set of fluorescent minerals, which glow in interesting colors only when they are being exposed to an ultraviolet light source.