A mirror does not swap left and right
Stand in front of the MIRROR and raise your right hand. The hand that goes up in the reflection is on the same side of the room as yours — it has not crossed over anywhere. Hold up a book and the writing runs backwards, which is the giveaway: what the mirror has reversed is front and back, not left and right.
The confusion comes from us. To picture someone standing where the reflection is, we mentally spin ourselves around, and it is that imaginary turn that swaps the sides. Anyone who doubts it can lie down sideways in front of the mirror, at which point the reflection obligingly swaps top and bottom instead.
You can feel a step that is a centimetre out
A flight of STAIRS is built to a rhythm. A French architect, François Blondel, wrote the rule down in 1675: two step heights plus one step depth should come to roughly the length of a walking pace. Get it right and people climb without looking.
The far more important rule is that every step in the flight must match. Once you are three steps up, your legs stop measuring and start repeating, so a single step even a centimetre taller than its neighbours is enough to catch a toe. Almost every trip on a staircase happens on the odd one out, usually the first or the last.
Clocks go clockwise because of shadows
Before there were gears there were sundials, and in the northern hemisphere the shadow of a sundial swings from left to right across the top as the sun crosses the sky. When European clockmakers started building mechanical faces, they copied the direction people already knew.
South of the equator the shadow goes the other way. Had the mechanical CLOCK been invented in Australia or Peru, clockwise would mean the opposite of what it means now, and every screw, tap and bottle cap that follows the same convention would turn the other way too.
The hall used to be the whole house
A medieval hall was not a corridor. It was the building: one large room with a fire in the middle where the entire household cooked, ate, argued, worked and slept, with the smoke finding its own way out through a hole in the ROOF.
Privacy arrived slowly and expensively. A room was partitioned off at one end for the owner to sleep in, the cooking moved away from the smoke and became the KITCHEN, and over several centuries the great room shed the BEDROOM, the BATHROOM and the rest. What survives of it is the HALL you now trip over shoes in on the way to the DOOR.
Room by room
The grid covers the lot: the KITCHEN, BEDROOM, BATHROOM and HALL, the STAIRS, DOOR and WINDOW between them, and inside a CHAIR, TABLE, SOFA, SINK, MIRROR, LAMP, CLOCK, SHELF, RUG, CARPET and PILLOW, with the ROOF on top and the GARDEN outside.
One of those is a small piece of poetry. WINDOW comes from an Old Norse word meaning wind-eye, which describes exactly what it was before glass became affordable: a hole that let in the light and the weather in equal measure. Homes are built from wood, brick, mud, stone and ice, and nearly all come down to a roof, a door, a hole for the light and somewhere soft to sleep.