But, weeks turn into months, months turn into years - and suddenly you find you are very attached, and when the years do eventually catch up it's not a comfortable experience.
We adopted Moff and Koop almost three years ago - it was alleged that they were already over 18 years old, so we really didn't expect to have them for very long.
They came as a double act, siblings who'd always been together but actually didn't much like other. Koop the fiesty one, Moff the more cautious (downright timid at times) - sometimes curled up together, other times finding their own places - or sometimes squabbling over the same place.
We were given instructions to let them enjoy their last few years - I think we've done that.
The years caught up with Moff today - hopefully Koop will stick around with us for a bit longer.