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Every so often an invention comes along which is so brilliant that we wonder how we ever got on without it. Fire is an excellent example. Ditto the flush lavatory and sunglasses.

A new reminder service established by an enterprising American might not beat, say, child immunisation as a contribution to human civilisation, but it has to be reckoned a contender., a website on which you can sign up to be sent an email alert that someone in your life might be approaching a particularly tricky time of the month, has received more than 100,000 enrolments. Men who enter the date and length of the last menstrual cycle of up to five women, will receive timely messages like "She's on yellow - tread carefully, fella". The founder of the free service, 28-year-old Jordan Eisenberg, says he hopes to launch it as an iPhone application soon - presumably a must for subscribers who don't check their emails regularly.

Rejoicing in the slogan "saving relationships one month at a time", the website should prove a boon for men who lose track of time while trying to work out what they did wrong two weeks ago.

Eisenberg says he has had his share of abuse from women who find the tracking of their menstrual cycles "a tad offensive" but he was far too diplomatic to suggest that the abusive correspondence might have been hormonally tinged. In any case, is an equal-opportunity service. Women can sign up the men in their lives, and give them a fighting chance of avoiding what one woman called "having the wrath of God unleashed upon them". Either way, soon no bloke will be able to say he wasn't warned.