One of my students came into school waving this ziplock bag. This student is one of those kids who always have a smile on their face and try their best. M was holding this ziplock bag and said laughing, "Mrs. Griffin, my dog ate my homework." Other students accused him of making it up, but he insisted. He related that he had gone upstairs for something and when he came back down, Sadie, his new puppy, had indeed eaten his homework.
So, although it is a trite saying, sometimes the dog really does eat the homework!