When we're young we're taught the imperial method since it's the customary system. Once we start getting into science classes it's all metric. Still, outside of science classes (and the military) we don't use it at all.

Which means we don't have a good working model in our heads of what a kg feels like or how warm a 30 degC day is...that sort of thing. I think most Americans have a pretty good feel for what a liter is since lots of drinks sell in that volume (and it's almost the same as a quart). But I think it'll be a cold day in hell before we cook in metric...customary units are pretty handy with home-style cooking since they line up pretty well with the size of stuff that's already in your kitchen (a cup, a teaspoon) and all our measuring and cooking equipment is in imperial. Actually, converting imperial measurements is a rotten mess, but using them in practice is pretty convenient. The sizes of most things just sort of intuitively makes sense on a human labor sort of scale (a pound of food turns out to be a nice filling meal, you can estimate inches with your thumb and feet with your forearm, 0 degF is colder than frozen water and just at the edge of tolerable, while 100 degF is just at the edge of tolerable on the other end of the scale even if water it nowhere near boiling, a pint is a good size for a beer, a yard is about a man's reach, an acre is about the size of a field a person can work in a day, etc.) -- there's some sense to them.

The military uses metric for most things however (due to constant international collaboration) and you'll find military people have a better grasp of the feel for most metric units, especially distance.

It's a bit like in England though where metric is taught, but people still colloquially refer to people in stones or distance in miles and beer in pints. Actually lots of countries still use their customary measurements [1] in certain niches, Koreans still use the pyeong 평 [2] when dealing with real estate for example.

I've been fortunate enough to travel quite a bit to metric using countries and have developed enough sense to quickly convert in my head. I know what a good room temperature is (22-24 degC), but I have no idea if I should bring a jacket when the weather is 15 deg C or not and the difference of 30 - 40 deg C always catches me by surprise. I have good feel for ml, cl and liters (lots of wine drinking), an okay one for kph, meters and km (driving and walking most places).

But g and kg continue to stupify me since I can't seem to find a useful frame of reference for those measures. I just end up mentally doubling all kg to pounds where it makes sense. I mean, what common thing weighs a kg? Everything at 1 kg just seems to be too heavy and to make use of it you end up sectioning it into grams, the liter is the same, too much liquid for anything useful, except I can usually split 1000 ml of wine with my wife over a long relaxing dinner and that's my reference point. And I guess that's the problem, I end up using fractions in decimal countries like I'd use them in the U.S. (most imperial units have handy fractions: 1/3, 1/4, 1/2, 1/12, but metric countries work better with decimals.

1 - https://en.wikipedia.org/wiki/Customary_units_of_measure#His...

2 - https://en.wikipedia.org/wiki/Pyeong