Una mano lava la otra. "One good turn deserves another." But this phrase is more commonly applied where two persons bespatter each other with fulsome and undeserved compliments. "Scratch my breech, and I will claw your elbow."You can read more commentary by Bland online, and here is more about Bland's book. The images comes from Cheezburger here and here. The poster is generated with AutoMotivator. Here is a full-sized view of the poster.