A conversation I overheard yesterday from the next room:
Hannah: Mummy? Do you know what porcupines are?
Nicola: Yes.
Hannah: What are they then?
Nicola: Well, they’re animals with lots of spikes, a bit like hedgehogs.
[Confused pause.]
Hannah: So why do people say “Don’t tell porcu-pines”?
Nicola: No, sweetheart, that’s porky pies. It means lies: don’t tell lies.
Hannah [penny dropping]: Ohhh!
Aside: Friends and family will know that porcupines (or one porcupine in particular) will have a special significance for Hannah one day. She just doesn’t know that yet. 
Fingers has his grasp already…
But why would the penny drop? Is rhyming slang genetically programmed? Already?