When a car gets stuck in the mud, the driver often tries to go forward/backward/forward, etc., until often times a 'rut' is formed. It is nearly impossible to get out of this rut, at least until the mud dries. When a
person is in a rut, he can go forward or backward, but it makes little difference.
[John has been teaching the same old stuff for so many years that it even bores him. He's definitely in a rut and won't try to change his methods.]