There I was, a terrified and impatient young parent, barefoot in the street in Marylebone, London in 1972, with my child having a *huge* tantrum.

He'd flung himself on the ground. I was leaning on the railings 10 feet away, ignoring him, with my arms folded and my face in a scowl, while he kicked and screamed and howled for sweets.

Every passer-by was glaring in disapproval at his snot-covered crimson face and angry shrieking and wailing, and then glaring at my equally angry but silent face.

Until a young woman came walking swiftly by. As she passed him she whipped out from her pocket a thruppeny chocolate bar, and handed it to him.

I waited for her disapproval, but she gave me a wide, friendly, sympathetic smile and kept going. Tears came into my eyes.

My son sat up, toddled over and pulled me down to sit on my lap, and carefully opened the chocolate. I wiped his eyes and nose - and my own - and put my arms around him as an after-sob shook him.

He took a bite of the chocolate - and then thrust the bar up to me to have a bite too.

I've always remembered that woman. A tiny act of kindness for her, but so huge in its effect. For years I carried one of those little bars of chocolate with me at all times, in case I'd be able to pass on the favour to some other mother!