One who reads is a reader.
One who writes is a writer.
One who speaks is a speaker.
But one who lies is a liar.

How did the usual e become an a?

Interesting question, though I don't know the answer. Perhaps it's the same reason why you spell fight f.i.g.h.t, and yet you spell bite b.i.t.e...that is, no good reason.

I've always wondered about the phrase "liar, liar, pants on fire." Obviously answerbag.com has no idea. However, there is an excellent discussion of the word "lie" (starting on page 15; Pants on Fire by Nick Humez) in Verbatim, and in one of the notes on page 17 it says that the origin of "liar, liar, pants on fire" is obscure, saying:

"The origin of the couplet "Liar, liar/Pants on fire" is obscure, its specific meaning even more so. Is the fate of the liar to burn in Hell, and thus by synecdoche his/her pants (i.e., the posterior inside them; cf. army drill sargeant usage, "Your ass is grass and I am the lawnmower")? Or are the fiery pants merely the consequence of predictable parental chastisement? Or does the verse imply that liars will or should meet their ends by being burned to death, as suggested by a Midwestern informant who knew this rhyme as a quatrain ending "Funeral pyre," whose third line she has unfortunately forgotten? Any readers who can shed additional light on this item should not hesitate to write in."

Kalleh, who will be away from her computer today, asked me to add the link to her Verbatim article.

I seem to remember from when I was really young, something like....

Liar! Liar!
Pants on fire!
Drag us through
the muck and mire.
Someone start
a funeral pyre.

It could have just been a local variation in our Detroit area neighborhood in the late 60's.

Can we add to wordnerd's list that one who advises may be an adviser, is sometimes an advisor, yet never an advisar?

I'm familiar with only the couplet for "Liar, liar...." But my favorite related expression is, "Ohhhhhhhhhh, Pinocchio...."

Interesting that you should remember that, Myth. Did you see that "funeral pyre" was mentioned in that Verbatim article?

I did not read the article, but I did read the excerpt from your post.