Lean

a leaning book leans on the next one. the last one holds them all.

a shelf of leaning books: slide the bookend and watch the sideways load add up along the row

A book that leans is not just untidy, it is pushing sideways on the book beside it, and that book passes the push along with its own share added on top. The push only ever adds, so by the time the row reaches the end, the last thing standing there is carrying every book at once -- which is why a shelf gives way at its end rather than in its middle, and why one small bookend can hold a whole row. Here that push is drawn as a hot line running up through the books, thickening at each one, and the brass bookend bends by exactly the amount it is being asked to carry. There is a second thing to see once you look: the row is a fan, and it has to be, because two books at the same angle either nest flush or never touch at all. Take a book off and the rest have to fan further to fill the same shelf, so the load goes up even though the weight went down.

Concepts: load, torque, physics.

An interactive you play with one thumb, on Wundr.