Boing...
...how did you type the word you want to spellcheck if you don't know how to spell it? The same mental process applies to looking in a dictionary.
What if you are offered by the spell checker "their" "there" "they're"; which one do you choose if you don't know how to spell it?
(A not too long trawl through the threads here will illustrate the weakness of spell checkers).
Ummmm is it "to" "too" or "two" long a trawl.........??
The Ancient Mariner