If a geoduck eats lots of beans, its head will grow larger. To a geoduck, beans are a brain food, so by eating beans, a geoduck’s head will grow incredibly large. Geoducks already look strange enough as it is, let alone with engorged heads.
As the geoduck eats more and more beans, its head gets bigger and bigger until finally the geoduck’s head explodes like an atomic (あたま) bomb. BOOM! Geoduck parts go flying everywhere and pieces of the geoduck’s massive head float to the ground as atomic fallout.