Gordon Freeman, the walking and lefting doom in the Combine's lines behind, isn't on the perex's picture for the Headcrab's fun. Again he took a place in today's logical game, where he is however playing only the second violins - Portal weapon, this is what is it about. In short, it places two connected portals, through which you can walk and throw things. With help of this original and mighty weapon you have to get through a lot of levels. And pardon, I got to go to hibernate myself until the time when would be this weapon invented on Earth. Have fun.