If there is a fully metric society, I have yet to hear about it.
If anyone claims to be fully metric simply ask how old they are.
Technically, seconds are the SI unit (minutes, hours, years are not).
I wonder if he is misrembering something I posted once. When Canada switched in 1974, morning show host Paul Rush came out with
Zero is freezing
Ten is not
Twenty is pleasing
Thirty is hot
The reason temperature is harder than the rest is that the conversion is not linear. (The technical term is that it is affine, meaning it involves change of base point followed by a linear one.)
I’m from Spain, so with a few exceptions fully metric. Some of those exceptions (arrobas, calling 400g a pound) are pre-metric leftovers, some are having learned to translate when I speak with people who don’t think in metric.
Still can’t think in Farenheit, tho.