Why is it so easy to weigh fish?
They have their own scales.
Fish Scales
Explanation
Explain Bear
Listen up, buttercup! The joke is a play on words. You see, fish "scales" are the little plates that cover their bodies, but "scales" is also a tool to measure weight. You probably didn't know what scales are since you are probably overweight, so that's why the joke made no sense to you.
igloo and you
Stolen