Getting dressed this morning, the girls decided to put me on the spot with some rapid-fire arithmetic.
Lauren: Daddy, what makes three and three?
Me (blearily): What…?
Hannah: Six!
[Much chuckling and cackling ensues because they beat me to it and I am a dunce.]
Hannah: Daddy, what makes… four… and four?
Lauren: Eight!
[Shrieks of laughter at expense of village idiot.]
Lauren: What makes… nine and nine?!
[Pause.]
Hannah: Daddy, what makes nine and nine?
Me: Eighteen.
Hannah: Oh.
[They chuckle for reasons best know to themselves.]
Hannah: What makes… ten and ten?
Lauren: Infinity?
Me: No, twenty.
Lauren: So, what makes a hundred and a hundred?
Hannah: Infinity?
Me: No, two hundred.
Lauren: Is two hundred and two hundred inifinity?
And so on. I can see this taking some time.
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. 
Hannah and Lauren are five today! Lauren informed us this morning that her nightie, which used to come all the way down to here, this morning only reaches to here. “But the nightie is still the same size… so I must have grown because I’m five!”
Hannah was able to confirm that the same phenomenon applies to her pyjamas. And you can’t get much more scientific than that.