Mrs. Jones was reading a letter at breakfast.  Suddenly she looked up suspiciously at her husband.

"Henry," she said, "I've just received a letter from Mother saying she isn't accepting our invitation to come and stay, as we do not appear to want her.  What does she mean by that?  I told you to write and say that she was to come at her own convenience.  You did write, didn't you?"

"Er, yes, I did," said the husband.  "But I couldn't spell convenience, so I made it risk."