When I see new parents and think "you wait...".
I was in a coffee shop this morning on my own and there were 2 new mums at the next table having a typical coffee. Both babies slept for the majority of the time and only woke briefly to be fed and then fell back to sleep again. During this time both mums went on about how this was the hardest thing they had ever done.
I often have to go to the local hospital and pass the labour wards. There is often some woman in the corridor leaning on her partner, breathing slowly and saying something like "ooo, this is a bad one".
I always smile politely and think "hee hee".