In reply to mike kann:
> Simple it may be but its fairly common to see people screwing it up. As a thing to teach kids, is it really that much of an issue to teach them about both metric and Imperial? Its hardly a massively complex job to do is it. As for conversion, as pointed out above, Hubbles mirror was screwed by european engineers not converting things correctly. SI may be globally accepted apart from one of the major economies in the world still uses it as a matter of course. Don't get me wrong, I think America should be using what the rest of the world use, but they don't and no amount of bleating is going to get them to change. Imperial is here for a good while yet so kids SHOULD learn about it.
I don't think anyone's going to argue that imperial units should ever be mentioned in school, but teaching the whole alternative system would be a waste of time. All a kid needs to learn is the fundamental principle of conversion of units of measurement, which is something they should be learning anyway whether you use imperial or make up your own arbitrary measurement system, and that only requires one or two examples.
In fact it could be argued that using your own arbitrary system would be better, as making stuff up tends to capture kids imagination, and if they get that you can define a "Samantha" as the length of Samantha's hair, and a "Ben" as the volume of water Ben's forearm displaces from a basin, and then measure and convert between those units and SI, then you know they're understanding the principle and not just learning conversions by rote.