Overheard in passing. Pillow knocking.

Two elder-aged women were standing talking in a hallway and as I walked by I heard one saying,  "… so I can take a nap and just tell myself to wake up in twenty minutes or so."  And the other one replied, "Well, if I want to wake up at seven in the morning I just knock my head seven times against the pillow before I go to sleep at night."