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

1. Double yolks are normally only made by younger chickens.

2. There’s a pretty simple process called ‘candling’ which is now done with bright torches pressed to the shell to illuminate what’s inside. The process is normally to check for fertilised eggs (wouldn’t happen when there’s no male chickens present) it also shows for double yolks.

I imagine more advanced egg factories would candle every egg and reserve the double yolks for higher-cost sales.