Two's a crowd?

I never really saw the point of tandem bikes (although I loved the three-seater they had on the Goodies way back when..). You both have to go at the same speed, in the same direction; the person on the back gets to stare at someone else's backside all the way (although admittedly not always a bad thing, depending); if you have a row you can't storm off and sulk on your own; and what the hell are you supposed to do when you need to go down the shops on your own, or want to go out when your co-rider is ill/busy/has left you?

I was pondering this on the way to work, prompted by seeing a bloke cycling past, on a tandem, alone. I wanted to shout out 'hey! you've lost someone!' and I'm sure I'm not the only one who got this urge. I suspect there are plenty of jokers around who thought it amusing to give in to this urge. And why not?!