You have 9 weights, but you don't know how heavy they are. You also have a balance scale, which you can use to compare any two groups of weights. For example, you could compare the first two weights with the next three, and it would tell you whether the first group was less heavy, equally heavy, or more heavy than the second group.
In the least number of measurements, how can you find two equal groups of four weights if at least one such solution is guaranteed to exist?