Pigeonhole Principle

Color principle

I went to a store which sells marbles in red, green, blue, and yellow, and I bought 100 marbles in total, and put them into a bag.

You'd like to reach in without looking, and take out a handful so that you're sure that you have at least 2 colors of marbles. You'd like to pick out as small a handful as possible which can still establish this guarantee.