A zebra doesn't eat apples
A zebra doesn't eat apples means you're being a total waste of time and need to stop being so useless.
You said you'd do the homework. You didn't. A zebra doesn't eat apples!
You told me you'd bring the pizza. You didn't. A zebra doesn't eat apples!
You said you'd help me. You're still here. A zebra doesn't eat apples!