Consider a piece of paper. If I pull on it in the right way it tears into two pieces. However, if I push those two pieces together, they do not spontaneously form a single piece. What happens at the microscopic level when the paper tears such that the process is irreversible?
My instinct is that in the fully formed piece of paper the fibers are strongly interwebbed, and so there is a large amount of (electrostatic?) energy stored at the boundaries between the fibers. To tear the paper, one must overcome this barrier, but I don't see what could have happened to make it difficult to re-introduce all those surfaces with one another. It almost seems like an entropy argument might work but I'm not sure how.