Spoiler :Grab a grape, and before anyone has a chance to see it, eat it. Then say that since the remaining grape is black, the one he picked must be white. Don't know, they might disqualify him for that.
Put one up then

In the meantime here's another one to tickle your fancy.
Bob and Alice were out wandering around enjoying the sites, when all of a sudden a volcano erupted. Some of the flow or ejected material struck them, but they didn't get burned. Why?