My favourite game is a first person logic game in which you are inside laboratory (Aperture Labs) and you go inside test chambers in which you have to solve logic problems. To help you solve the problems you have a portal gun, this gun can create 2 portals at the same time, a blue portal and an orange portal. This portals are connected, meaning that if you place a blue portal in point Y and an orange one in point X, you can travel from point Y to point X and vice versa through the portals.
Here is an imge explaining how portals work
In order to complete a level you must solve logic challenges to open the door to the next level.