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Research and Development is a single player Ep2 modification that purely focuses on puzzle solving and riddles. Not to say that there is absolutely no sense of danger; because there's plenty of it in this mod. Despite being released years ago, Research and Development still remains one of the best Half Life 2 modifications in the pool and I definitely recommend anybody who hasn't tried it out yet; to give this one a go.
That said, the large amount of puzzles are very clever, intuitive, and creative. While some are frustrating, each is extremely satisfying, if not hilarious, to solve. The game is also much more than completing a puzzle, moving on to the next room, and completing the next puzzle. All areas in the game feel intertwined and related to one single breathing world. Many scenarios in the game are also "action packed" to say the least. Despite that there isn't a single obtainable gun, (by gun, I mean a gun that shoots bullets) the game will still throw combine soldiers or helicopters at you; as well as roller mines.
Of course, there's a very logical and fair way for dealing with all the enemy encounters in the game. Furthermore, it will take some brain power, and maybe some fast reflexes, but nonetheless; this mod isn't your typical puzzle game.
...Of course, if you hate puzzles; you should probably look somewhere else. But hey, if you had to start somewhere, start here.
