What doesn't kill us makes us stronger.

One Ma used a lot when we were kids, and I was surprised that it wasn't common enough for others to know it when I went away to college:

"Two stubborn birds of equal strength can stretch a worm to any length" - in other words, "Okay you two, quit bickering and let it rest. No one is gonna win, so give up!"

Another oldie, this one from Grandma Peterson: "You aren't made of sugar, so you're not gonna melt ..." to any child (or pet) reluctant to go outside or off the porch when it was raining.