I have an opinion: I believe "sixths" is the most difficult word in the English language for anyone to pronounce, even native speakers.

I came across the word in a puzzle recently, and have been turning it over since.

Sixths.

The word starts at the front of the mouth for 's', zips to the back with 'k' then the front with 's' and the tongue a little back (x), then the tongue gets in on the more forward act behind the teeth for 'th', then moves back for another 's'.

It's easy to say words ending with 'ths' like "months" or "births". Even "sixth" is manageable. In fact, we're liable to fudge the 'th' a bit by not really saying it, merely hinting at it. But putting that final 's' on the end of "sixths" is a tongue twister.

There's no real way to fudge it, with the 'th' between two 's' sounds.

I guess we're most likely to just leave out the 'th' and say something like: "the glass is 5 sixs full". Or a smart person would say (less accurately) "about 80%".

Crazy language, English!