Cuckoo's egg (metaphor)

Okay, let's talk about cuckoo's egg. You know how there is a bird called cuckoo? These birds lay their eggs in other bird's nests and trick them into hatching the cuckoo bird's egg. Once the egg hatches, the cuckoo bird grows faster and stronger than the other birds in the nest because it's bigger and takes all the food. Eventually, the cuckoo bird kicks all the other birds out of the nest and takes over.

Now, let's apply this to something else. Sometimes people use the term "cuckoo's egg" to describe something that doesn't belong somewhere but was put there by someone else. For example, if someone sneaks a virus onto your computer and it starts doing bad things, that virus is like a cuckoo's egg because it wasn't supposed to be there but it got planted anyway. Or, if someone plants evidence to frame someone else for a crime they didn't commit, that evidence is like a cuckoo's egg because it doesn't belong there but was put there to trick others into thinking something else.

So, to sum it up, a cuckoo's egg is something that doesn't belong but was put there to trick others into thinking it does. It's like the cuckoo bird tricking other birds into raising its baby instead of their own.