入口(rù kǒu) - entrance (building) in Mandarin

入口 (rù kǒu) is 入 (rù, "to enter") plus 口 (kǒu, "opening, mouth"), literally "the opening you enter through" - the standard word for the entrance to a building, a car park, a station or a website. It behaves as an ordinary noun: 入口在哪里 (rù kǒu zài nǎlǐ), "where's the entrance?".

Its exact opposite is 出口 (chū kǒu, "exit"), built the same way from 出 (chū, "to exit") plus the same 口 (kǒu). Learning the pair together is efficient: same second character, opposite first character, and the meanings mirror each other precisely, the way "entrance" and "exit" do in English.

At a venue with a ticket gate, 入口 (rù kǒu) is often where you show your 门票 (mén piào, "entrance ticket") before going in - the two words appear side by side on signage constantly.