The who-dun-it is a classic movie genre, as well as an example of bad grammar getting absorbed into mainstream usage. There are plenty of these murder mysteries coming out today, especially if you include TV shows like "Law and Order" or "CSI," or even medical who-dun-its (what-dun-its?) like "House." But the genre used to be much more popular on the big screen, especially in the 30s and 40s. If you wish to familiarize yourself with some classics of the genre, just lock yourself up with one of these four classic who-dun-it movies that will leave you guessing till the end until you figure out the mystery.

"The Thin Man"

Perhaps the pinnacle of the who-dun-it genre, "The Thin Man" features detectives Nick and Nora Charles trying to solve the murder of the titular character. But don't watch this expecting to see a hardboiled murder mystery: This is sparkling Hollywood comedy at its finest, with the delightful onscreen chemistry of William Powell and Myrna Loy as the main attraction. In other words, it don't matter who done it, just that it gets done.

"Charlie Chan at the Opera"

The thing about who-dun-its is you can never assume you know exactly who-dun-it, even if it seems obvious. Take Boris Karloff in "Charlie Chan at the Opera" everyone but the titular super-sleuth assumes straight ahead that just because he's an escaped maniac from an insane asylum returning to an opera house to exact revenge on those that locked him up and tried to burn him to death eleven years before, that he's automatically a murderer. But, like Detective Chan, we viewers of who-dun-its have to watch out for red herrings. Also knives. And bullets.

"Mr. Wong, Detective"

Here's a fun bit of movie trivia for you: The detective hero of this movie (and several sequels) is played by the bad guy (or at least the ostensible bad guy) in "Charlie Chan at the Opera," Boris Karloff. As always, the who-dun-it rises and falls on the strength of its lead detective, and Karloff is no slouch. Sure, Mr. Wong is just a skinnier (and much creepier) Charlie Chan knock-off, but as knock-offs go, this is a lot of fun.

"The Girl Who Knew Too Much"

The who-dun-it movie wasn't the exclusive domain of Hollywood. In Italy, they took the who-dun-it formula, added lots of shiny blades, gushing blood, and gruesome kills, and called it "giallo." Here is a who-dun-it that many scholars consider to be the first giallo movie. As the title suggests, there's a healthy bit of Hitchcock influence going on, too, so you know this is one entertaining who-dun-it.

These four classic who-dun-it movies are fun for the mind, the brain, and your general head area. Can you figure out who murdered whoever else before the main character does? If so, congratulations, a future in law enforcement may be for you. Or at least a future in watching old movies.