What explains a green laser reflecting off an orange ping pong ball producing bright yellow? I shined a green laser pointer at an orange ping pong ball today and saw a distinctly bright yellow color.  
At first I thought about it and reasoned that green=(yellow + blue) hits the ball which reflects orange=(yellow + red) thus yellow is reflected.
Then I thought, wouldn't this mean that a prism would break a green laser into yellow and blue?  A quick search confirmed this is obviously not the case since laser light is a single wavelength, not an additive combination of primaries...
What then is a good explanation for this?

 A: Laser light from a green laser pointer is essentially monochromatic. I demonstrated that in this earlier answer which shows how to measure the wavelength - in the process of that experiment you see the spectrum consists of a single peak.
I hypothesize that your orange pingpong ball is fluorescent (this is often done to make the ball more visible: see this article for more details. As that article mentions, white balls are used in tournaments, where the table, floor and clothing are dark-colored, lighting is sufficient, and white will provide better contrast).
As the green light has a shorter wavelength (higher energy) than the emission wavelength of the phosphor, you excite the phosphor in the ball - and it then emits the longer wavelength of light.
You see a similar thing if you use a 405 nm (almost invisible, near UV) laser pointer and shine it at a piece of white paper. The paper contains compounds that fluoresce, and the resulting beam suddenly goes from "barely visible purple" to "bright blue".
