If a triangle has a 4 meter side and a 9 meter side, between what lengths could the last side be?

I have the answer, but I can't figure out why/how.

Thanks!

it has to be long enough so that combined with the 4 m side, it can reach the end of the 9 m side to form the triangle

it has to be short enough so that the 9 m and 4 m sides can close up the triangle

(9 - 4) < s < (9 + 4)