Two Hipopotami were standing together in the middle of the Kalahari Desert when one one turned to the other and said " Funny, I could have sworn it was Tuesday"

You have to be really with it to understand the sublety of this one, but it has a similar theme to the Ostrich Joke:
" an ostrich arriving home one evening, enters a clearing where a hundred other ostriches have their heads stuck in the sand. "funny" said the first ostrich " Where's everyone gone"