Science fiction author Robert A. Heinlein once noted that "there is no free lunch."

This is definitely true of the Half-Life series, currently available for free on Steam. To help players prepare for the launch of Half-Life: Alyx, Valve's upcoming VR game, the company is offering most of the Half-Life series for free through March in preparation for the launch.

The games aren't hard to get, but remember, if you don't want to pay for them, you have to finish them before Valve's grace period expires.

To begin with, there are seven games available for free. They are:

To get the games, simply open the Steam client, navigate to the game you want to play, and hit "Play Game." The rest is easy.

For those who haven't played it before, all Half-Life games are first-person shooters, interspersed with physics-based puzzles.

The first game is about a physicist who must fight off an alien invasion; Opposing Force tells the same story from the perspective of a military operative called in to cover everything up. Blue Shift offers a third point of view, and involves the manipulation of one of the ill-fated lab guards.

Half-Life 2 is perhaps the pinnacle of the series, advancing the story 20 years and pitting protagonist Gordon Freeman against both aliens and the hostile fascist regime that has taken over Earth. Episodes 1 and 2 continue the story, but then a cliffhanger awaits: hopefully Alyx will resolve it.

Team Fortress Classic is the granddaddy of role-based multiplayer shooters and is worth playing as a historical curiosity.

Buying all of these games usually costs about $50. Keep this length of time in mind if you plan to play the entire series; March 2020 is when Half-Life: Alyx will be released, at which point Valve will (probably) make the games pay again.

The good news is that Half-Life: Alyx promises "two months" until the Steam client is currently out of promotion. If you want to continue playing after that, you'll have to shell out some money, but at that point you may be ready to move on to Half-Life Alyx, which costs $60 and requires a VR headset.