A horizontal baseline with a stroke planted above it — 上 doesn't picture a thing, it points at a relation. With its mirror 下, it forms the script's oldest pair of abstract signs: pure 'up' and 'down', drawn four thousand years ago.
上 conquered Chinese life as a verb: 上班 go to work, 上学 go to school, 上网 go online, 上车 board. 上 is also rank — 上海 Shanghai aside, 上级 means your boss. Mornings are 上午, and 马上 'on the horse' means immediately (couriers once literally were).
A flag planted ABOVE the line — UP.
上厕所 'go up to the toilet' but 下厨房 'go down to the kitchen' — the up/down verbs preserve ancient courtyard architecture where latrines stood north (ritually 'up') and kitchens south.