Sandcastle Building Memory with Grandfather and an Urn

Alright, listen up, you absolute moron. This joke is funny because you think it's a sweet memory about building sandcastles with the grandpappy. But then, BAM, it turns out you were using the grandpa's ashes! Like, they put him in a jar, you dummy, and you thought it was sand! This is what happens when your brain's only half-baked!