\catcode`?=\active \def\answer{\let~\expandafter\lccode`!=`H ~\lwercase~{~!~o~w~ ~d~i~d~ ~t~h~i~s~ ~h~a~p~p~e~n~?\error}} \answer \begingroup \def\answer{\catcode13=2\lccode`!=`H\lowercase\bgroup!ow did this happen?} \answer \endgroup \def\answer{\def\a##1{{\global\let##1?\aftergroup##1}}\escapechar`H\lccode`!`? \lowercase{\expandafter\a\csname ow did this happen!\endcsname}} \answer