‘Solid’ glass can actually be better described as a slow-moving liquid. The particles in glass can move, just like in a regular liquid, because they are not fixed in an orderly structure. In glass, this movement is so slow that it cannot be detected during a person’s lifetime or even over the course of several generations. Medieval window glass which has survived to the present day is often found to be thicker at the bottom than at the top. This clearly shows that glass flows.