A: Why did you quarrel with Tim over such a trivial incident?

B: I don’t mind my roommate being a bit messy, but leaving dirty dishes for me to clean up is where I draw a line in the sand!

Note: This idiom means “to set a figurative boundary that someone or some group refuses to cross or beyond which no further advance or compromise is accepted.” Perhaps Spartans at Thermopylae did draw a line in the sand with their swords signifying no retreat, and a Roman general reportedly drew a line in the sand with his sword daring an enemy to cross it and so start a war against Rome. But there is no convincing evidence of the idiom dating back to those ancient times.