It's because you understand that programs, when you write them, are frozen at time zero, that it's s-expr tentacles are ready to accept inputs, and when they ge

It's because you understand that programs, when you write them, are frozen at time zero, that it's s-expr tentacles are ready to accept inputs, and when they get an input, they contract in computation and, for long lived programs, emit new tentacles that no programmer ever wrote. (If you can write reliable tentacles and dress them up so that people want to touch them frequently, you can become very, very rich.)