There is no strict rule that requires you to play Half-Life 1 before Half-Life 2, but if you want to truly understand and appreciate the storyline, playing them in order is advisable. Half-Life 1 provides a foundation for the universe, its characters, and key events. It follows the story of Gordon Freeman, a theoretical physicist, during an alien invasion. Some of the puzzles and mechanics introduced in Half-Life 1 are also carried through the sequel, so getting familiar with them can enhance your gameplay in Half-Life 2. When you move on to Half-Life 2, you’ll notice that it builds upon the original game in terms of storytelling, physics, and gameplay mechanics. Having the context of the original game can greatly enhance your appreciation for these improvements. Half-Life 2 continues the story of Gordon Freeman in a world now dominated by the alien Combine Empire, offering more complexity and depth. However, if your main interest is better graphics, advanced mechanics, and a more involved narrative, you may want to start with Half-Life 2. Valve, the developer, designed Half-Life 2 with enough in-game context so that it can be played and enjoyed without prior knowledge from Half-Life 1. There are also many summaries available online if you want to get caught up with the story quickly. In conclusion, for a comprehensive understanding and more profound experience, it would be best to start with Half-Life 1. But if graphics and mechanics are your priority, consider starting with Half-Life 2. Either way, both games are landmarks in the history of gaming and offer thrilling experiences.