They started as same word, but their meanings have drifted apart over time.
What's with his feathered cap?
Why both words are equally good for you.
Why does it sound strange to say 'funner' or 'funnest?'
You don't have to end all your adverbs in -ly to talk right.
We're gonna stop you right there
A simple way to keep them apart. (Most of the time.)