But physical pipes run in parallel too! If you have a pipe full of water, and you push some more water in one end, there is an instant effect at the other end.

But physical pipes run in parallel too! If you have a pipe full of water, and you push some more water in one end, there is an instant effect at the other end. If it's empty, then sequentiality is introduced because it takes time for the water to flow through the pipe. That's how a Unix pipe works – (the processes at) both ends of the pipe come into existence at the same time, but if the right side is waiting for input (the pipe buffer is empty), it won't do anything until the left side has produced output.