eli5 Eggs in the supermarket state on the box “Double Yolk”. Without cracking them, how do they know….?

Question

Apologies if this is ridiculous and should be in no stupid questions but it just struck me

In: 2748

Answer 1

They’re bigger. A single chicken will lay pretty consistently sized eggs, with only a few grams variation. Then one day it lays an egg 20g larger than usual — that’s a double yolk. Say you’ve got 60, 61, 58, 60, 62, 59, 60, 80. The last one is a double yolk.