Couldn't Spell

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, I couldn't spell 'convenience,' so I made it 'risk.'"