If real estate is all about location, location, location, then there's one thing I know I'll always think about when it comes to making video games; it's collision, collision, collision. What should hit what, when, where, why, and how. It's how things happen, whether it's rewarding or damaging, things have to happen when they hit each other. Especially if you want to object pool gunshots, enemies and obstacles, otherwise memory will take a hit, and the frames will begin to suffer. Once the bullet collides with the alien, both can be sent back to their respective pools to meet once again on the battlefield.