Recently I was in the hospital for five days due to a fever. Because the doctors were not sure as to the cause of the fever, I was placed in a private room. This room had a small ancient TV which gave a horrible picture; but as I don't watch TV, that did not bother me. Due to my MS (I have the crippling disease Multiple Scleroses) I couldn't get up or move about. And I was so weakened by the fever that I could not lift my arms from the bed (so no reading or listening.) I was, you might say, trapped in that bed in that room for five days.
Directly in front of me on the opposite wall there was an analog clock. After awhile, I began to devise geometry problems associated with the clock. Little by little I felt myself drawn into a fascinating little mathematical universe. In those five days I did not come close to exhausting the kinds of problems one can encounter in this closed little system: I and others continue to discover new things more than a year and a half later.
I found that all the problems I have devised can be done in the head (I myself must do them that way because I can no longer use my hand to write.) The only mathematics involved is the use of fractions and simple algebraic equations.