Don't judge a book by its cover

There is not a clear origin for this common phrase but there are a few ideas. 

One source says the expression goes back to at least the mid-19th century, as seen in the newspaper Piqua Democrat, June 1867:

"Don't judge a book by its cover, see a man by his cloth, as there is often a good deal of solid worth 

and superior skill underneath a [???] jacket and yaller pants."

Other ideas are found here.

The expression is meant to convey that often what someone or something looks like isn't representative of the truth of who they are or what it is. Like Susan Boyle, below. Simon dismissed her because of how she looks...until she opened her mouth.