Two Eskimos sitting in a kayak were chilly, but when they lit a fire in the craft it sank.
This proves once and for all that
y* c* h* y* k* a* h* i*, t*.
Since only the first letters of the punchline's words were left, one might miss the pun...
H e l p !
(In reply to solution
It's a great pun which originates from that hilarious and erudite BBC radio show, My Word. It ran from the late 1950s until 1990 and the inimitable Frank Muir concocted that particular crazy story to explain the "real" source of the "cake and eat it" proverb.
PS Anybody still remember radio ...? :-)