What do you call a cow that eats grass?
A lawn mooer.
Listen here, Lucy, you aren't the sharpest tool in the shed. This joke is about a cow, right? And what do cows eat? Grass, obviously. And what do people use to cut grass? A lawnmower, duh! So if a cow is eating the grass, it's like a “lawn mooer”. Get it? It's a pun!