Phrase for a problem that seems impossible, but actually has a simple and obvious solution?

Daniel M. Russell poses what he claims is a deceptively simple brain teaser in his blog:

What short 4-word idiomatic phrase (in English) captures [the] idea of a problem that seems impossible, but actually has a simple and obvious solution?

"Deceptively simple brain teaser" is the best I could come up with, but I don't think it's particularly idiomatic.

UPDATE:

So, the originating blogger has posted his intended answer. I would agree that there was no stand-out candidate amongst all the plausible suggestions, so kudos to all those who found a solution.

One of the comments on that blog suggests "An egg of Columbus". From the description here, http://en.wikipedia.org/wiki/Egg_of_Columbus, it certainly seems to fit the bill.

It's to "cut the Gordian knot."

How about

Elementary, my dear Watson

Of course this was never uttered by Holmes in any of Doyle's published works.