He bought fourteen of them: seven for him, seven for his wife.
'What?' he said. 'They were on special offer.'
'The only problem, darling,' his wife pointed out, 'is that they're identical. How will you know whether or not the bristles you're running over your face in the morning were lathering my armpits eleven hours earlier?'
He pondered this, stroking his (smooth, stubble-free) chin. 'I shall label them' he announced, and immediately left the house in search of a stationer.
That afternoon, he lined up the brushes in two neat rows: a mini-platoon of brushes bearing a small red sticker, facing an equal number of green-stickered comrades.
His wife nodded approval. 'Red and green. Like it,' she said. 'Which am I?'
'Whichever you like,' he replied. 'I avoided pink and blue,' he added. 'I know how you feel about pink and blue.'
'Quite right. Thank you.' His wife picked up a green brush. 'I'll take red,' she announced.
Neither the stickers nor the rows lasted long: wet hands rapidly wore away the coloured dots, and his wife had never been one for keeping things tidy. So he was never quite sure where his morning brush had been. And, despite several furtive Google searches, he never worked out how common it was for women who objected to pink and blue also to shave the many, intimate body parts that his wife did.