In waves combine so that the resulting wave is bigger than the largest of the original waves?

Explanation:

When two waves are in phase, their peaks and troughs line up.

They undergo constructive interference, and the amplitude of the resulting wave is larger than either of the individual waves.

In the diagram below, the two red waves are in phase, and they combine to give a new wave (green) with a greater amplitude.

a corroding iron, if that's one of your choices