Phoebe has never had to be alone: even when I've been in the kitchen, surrounded by bubbling pots and sharp knives, I've always been able to leave her playing happily with her sister.
This weekend though reminded me sharply of Ellie's first year: every time I had to leave her alone the eight-month-olds' separation anxiety kicked in, producing a panicked, sobbing baby girl, perhaps even more distressed simply because she has always had a sister to rely on in the past.
Fortunately I always hurried back so hopefully the all important lesson -that just because Mummy goes away it doesn't mean she's gone for good- will soon be learned.
Meanwhile I started pondering the differences between Phoebe and Eleanor.
It isn't that I hadn't noticed any differences before, in fact when Phoebe was first born all I seemed to see were differences: she was about twice the size that Ellie was at birth, she fed and slept happily, gained weight and grew easily*, she was nothing like her sister at all, even in appearance**.
But as I got used to Phoebe the differences became far less pronounced: these days she even looks much as Eleanor did at this age, just a little larger.
So it took a little thought to notice just how different these two are.
To begin with there's Phoebe's reaction to music.
Play a decent piece of music to Phoebe and she will sit transfixed.
In fact I gleaned the time to type this by putting on Turandot in the background***.
To Eleanor the correct response to music has always been to get up and dance, or to try to sing along.
Even before she could walk or talk she would start to wriggle and wave to the music, or start to yowl along until it finished.
Phoebe, however, listens.
Sometimes she'll try to join in, drumming or shaking a rattle in very inaccurate time, but mostly she just listens, she will even let me sing her to sleep.
Then there's the way Phoebe wakes up happy whereas Eleanor always gave the impression that she was thoroughly distressed to find herself back in the same world she left when she went to sleep, in fact Ellie is still sometimes grumpy on waking up, Phoebe, meanwhile, wakes up wreathed in smiles.
Phoebe can't walk yet but Eleanor was walking happily at six months, Eleanor's first word was "Cat" but Phoebe's was "Daddy" (neither thought I was worth remarking on), Phoebe is cautious around strangers whereas Eleanor would and will smile at anyone, Eleanor grabs any new toy and dives straight in but Phoebe will study it from every angle before tentatively trying something, like a scientist testing a hypothesis.
Does all this mean that Eleanor is doomed to be a gregarious pessimist, while Phoebe will be a happy intellectual?
I severely doubt it.
But it does show that, despite all their similarities, despite sharing their genetic makeup, their upbringing and -reluctantly- their toys, they are and always will be individuals.
And it means that for all my experience with Eleanor, I still can't count on anything.
* My baby is not a rhinoceros, she is perfectly average, Eleanor was born tiny and got tinier.
** Eleanor looked like an unfledged crow or possibly an alien, Phoebe just looked like a baby.
*** This sounds terribly pretentious but if I'd put on the Teletubbies it would just be ordinary bad parenting.