A woman working in a laboratory is responsible for keeping tissue cultures alive. So that she won't forget, she writes "feed cells" on her calendar. One day she noticed that someone had scribbled in "take cells for a walk."

By the end of the month, a number of anonymous reminders had been added:
Take cells to Disneyland...
Cells on vacation...
Cells back...

On Yom Kippur, someone added, "Jewish cells get the day off."