Working on a crossword puzzle requires stamina. After 15 minutes of work, the clues become more difficult to solve. After a restful break, where the mind is allowed to recharge, the answers appear more easily.

There is nothing unique about the above description. Everything in life follows that pattern. Things are easier when one has more energy. This goes for physical and intellectual exercises. It is easier to observe the effects of fatigue on physical tasks. For intellectual tasks, flagging energy is more ambiguous, and one can interpret the effects as an intellectual difficiency. Crosswords can make one feel dumb, but often, that feeling is multiplied by fatigue.