Why are there some parents who walk all over us, who take and never give? Who simply sponge.
Generally, it all works fine. Reciprocal arrangements - I'll pick up on Tuesdays if you can manage Thursdays - make life easier. Taking it in turns to ferry children to after-school activities saves on time and petrol. But there's always the odd grabbing parent who pushes the whole thing too far.
It was seven o'clock. My friend (let's call her Jane) had picked up her five-year-old (Tom) and his friend (Charlie) from school, had given them tea and had kept an eye on them while they played. But now the boys were tired and tearful. Charlie's mother had said she'd pick him up at six. Where was she?
At a quarter to eight, the doorbell rang. There was Charlie's mother, all smiles.
'Oh, it was so much easier doing Sainsbury's without him,' she said.
Over the sound of wailing five-year-olds, Jane looked at her, confused.
'And I was able to take it all home and unpack,' said Charlie's mum.
Jane glanced up at the clock.
'I won't stay, if that's OK,' said Charlie's mum. 'We're going out tonight.'
Thank goodness for small mercies, thought Jane. I could have been babysitting till midnight.
'Every day for a whole year I walked my children to school with the little girl down the road,' says a friend who prefers to remain anonymous. 'It was fine. I wanted to say goodbye to my lot in the playground, and we were passing this little girl's door anyway. But then one day I had an early morning meeting. So I asked my neighbour whether I could drop my children off at hers, and she could take them all instead. She said, wouldn't it be easier if they just walked on their own? I was speechless. My youngest is only four.'
Sometimes the sponging is blatant. 'There was a single dad at our school who was completely shameless. He said to me once, if you're making brownies for the cake sale on Saturday, could you cook a few extra and say they're from me? It was ridiculous. We were both working. He just seemed to think his time was more precious than mine.'
Sometimes you're manipulated with great finesse. 'My friend said her relationship was going through a bad patch so I offered to have her boys on Saturday afternoons to give them a break. A month later, I found out her partner was working in Germany.' Small favours become huge commitments. 'I said I'd bring her daughter home with us from swimming on Mondays. A few weeks later, she said, if you've already got her at your house, could you keep her until eight? I'd like to start going to yoga.'
The problem is that it's a basic instinct to help other people. If you're asked for a favour, you're likely to say yes - especially if you're already so distracted by urgent tasks that you're buzzing round like a fly at a picnic. The difficulty comes when a sponging parent hitches a free ride. There you are, struggling through the thick gloop of everyday life - and you don't even realise you're dragging someone else's weight behind you.
We're not talking emergencies here. Obviously, you don't even ask questions when there's a child with a temperature of 39.9, or someone's grandmother has been rushed to hospital. In those situations, you help out, and gladly.
But you shouldn't have to put up with a sponging parent who rearranges your life just to make his or her own that much easier.
So what can you do when the penny finally drops that she's asking you to step in not because there's a family crisis, or an unexpected drama, or sudden illness, or a desperate panic - but just because she thinks of you as a form of cheap childcare?
Here's a simple exercise.
Try shaking your head very slowly from side to side.
What to say to a sponging parent
I can't look after little Johnny tonight because...
I'm catching a plane to Uganda
We've all got dog flu. It's like swine flu but worse
I've failed my CRB check and I'm not allowed to look after anyone else's child
They've discovered an unexploded bomb in the kitchen
It's the full moon and I usually turn into a werewolf
We've got a major flea infestation
It's against my religion
I have anger management issues
My boss is going to call an unexpected meeting
I'm judging The Apprentice