Let me guess - mini hot water bottles? Don't be primitive.

Go on, then. It's a heating element in an insole, connected to hi-tech rechargeable batteries.

Bit uncomfortable, having batteries in your boots. Don't be a chump-the batteries clip behind your heel with a lead down into the boot.

Keeps the tootsies toasty? Quite. Ideal for the coming winter. And you can fit them in anything from ice skates to waders.

How soon does the juice run out? Twelve hours on the lowest setting of 90F, 2 hours on the fourth setting of 154F.

Then you chuck 'em away? No, you charge them overnight on a special recharger, up to 1,000 times. The makers say the average lifespan is five years.

Could only be invented by Americans. This time you're right. But they were upgraded recently and about 200 are sold in this country every year.

Arm-and-a-leg job? £99.95 including the recharger. £29.95 for a replacement battery pack, £3.95 for a ski boot mounting bracket for the batteries.

Can I fit them myself? Yes, but the shop will do it for you. Either the insoles are trimmed to fit your foot, or you buy them without insoles and attach the elements to your own insoles.

Where do I get them? Ski specialist Snow + Rock (01932 581228, snowandrock.com) is the UK agent: three shops in London, others in Chertsey, Birmingham, Bristol, Hemel Hempstead and Sheffield.