The Misspelled Invitation

Mrs. Foggybottom was reading a letter at breakfast. Suddenly she looked up suspiciously at her husband. "Harry," she said, "I've just received a letter from mother saying she isn't accepting our invitation to come and stay, since we do not appear to want her. What does she mean by that? I asked you to write and say that she was to come at her convenience. You did write, didn't you?"

"Er, yes, I did," said the husband with a smile, "but I couldn't spell 'convenience,' so I changed that last word to 'risk'."