门槛
mén kǎn · HSK 7
threshold
🚪 门 (door) + 槛 (sill) = the raised threshold board under a door; figuratively the knack to get past it
门 means door or gate; 槛 is the wooden sill at its base that you step over. The literal doorstep became a figure for a hurdle to clear, and then the clever trick or knack for clearing it cheaply.
门 — pictograph of double-leaf gate
门 is a pictograph showing a pair of swinging doors hung in a doorway, originally drawn as 門 in oracle-bone script with two clear leaves. The simplified form 门 keeps the same outline with fewer strokes. It still means 'gate' or 'door' and serves as a phonetic in characters like 们, 问, and 闷.