所 pairs a door 户 with an axe 斤 — by origin, the sound of chopping, borrowed early for 'place': where the work happens. The axe by the door marks the spot.
所 marks places and abstractions alike: 厕所 (toilet — the place every traveler must ask for), 所以 (therefore — 'that by which'), 所有 (all — 'that which is had'). Institutional China lives at 所: 派出所 (police station), 研究所 (research institute), 诊所 (clinic). Ask 厕所在哪儿? on day one; appreciate 所以 by week two; by month three, 所有的汉字 — all the characters — are yours.
An axe 斤 working by the door 户 — the PLACE where things get done.
厕所在哪儿 ('where's the toilet?') is statistically among the first full sentences every learner deploys in China.