口(kǒu) - mouth; measure word for members of a family in Mandarin

口 (kǒu) draws a mouth, quite literally: the character is an ancient pictograph of an open mouth, and the shape has barely changed in three thousand years. Used on its own it means mouth, as in the everyday phrase 开口 (kāi kǒu), "to open one's mouth", meaning to start speaking.

Where this word list actually tests 口 (kǒu), though, is as a measure word (量词, liàngcí) for counting people in a household. Chinese does not use the general-purpose 个 (gè) for family members; it uses 口 (kǒu) instead. 你家有几口人？(nǐ jiā yǒu jǐ kǒu rén) asks "how many people are there in your family", and the natural answer is something like 我家有五口人 (wǒ jiā yǒu wǔ kǒu rén), "there are five people in my family". The logic behind the choice is homely rather than formal: a household is counted by the mouths it feeds.

Keep the two uses separate in your head. 口 (kǒu) counts family or household members specifically, not people in general - a class of thirty students is 三十个人 (sānshí gè rén), never 三十口人. Get that swap wrong and a fluent speaker will still understand you, but it will sound distinctly foreign.