I could not think of the finger joint...thought to myself that I'll just have to look up that word for finger...knuckle. Started to make a new tab...realized knuckle was the word! Ain't it great to get old? Not.
You're backside doesn't need to touch your heels, in church people kneel while leaning forward against the pew in front. And some do this until they are very old indeed so it's not that hard.
Unrelated trivia: the famous Notre Dame football coach Knute Rockne pronounced his name "Canute", but say it like that now and you will be considered stupid!
Never heard of knoll and knell, but obviously knell is somwehat known looking at the scores. I kept trying knocker for the dooropener (and thought, well it cant be key...). The only other word I could think of was doorhandle, so that was ruled out..
Most people have heard "knell" used in a common phrase but probably confused it with "nail"... "death knell". There is a similar phrase around death/doom that does use "nail" that probably contributes to the confusion, "nail in the coffin".