A prisoner in jail receives a letter from his wife:

A prisoner in jail receives a letter from his wife: "Dear Husband,
I have decided to plant some lettuce in the back garden.
 When is the best time to plant them?" The prisoner,
knowing that the prison guards read all mail,
replied in a letter: "Dear Wife, whatever you do,
 do not touch the back garden. That is where I hid all the money.
" A week or so later, he received another letter from his wife: "Dear Husband,
You wouldn't believe what happened, some men came with shovels to the house, &
dug up all the back garden.
" The prisoner wrote another letter back: "Dear wife, now is the best time to plant the lettuce."