There are four red balls and four blue balls in the box. If you want to touch two balls of the same color, how many balls do you have to touch at least?

There are two colors. If you touch three balls, you can make sure that the two balls are the same color.

As long as the balls are 1 more than their colors, two balls can be guaranteed to be the same color.

Make sure that the two balls are the same color: number of balls in contact = color type+1.