Here's the answer:
“口”
(kǒu)
Watch a real native speaker say it:
Here's how it sounds in a textbook:
Fun fact: the hundreds of thousands of people learning Chinese with Memrise get this phrase correct 95.12% of the time!
(kǒu)
Fun fact: the hundreds of thousands of people learning Chinese with Memrise get this phrase correct 95.12% of the time!