Ever wondered what exactly is beyond the levels in Half-Life 2? Grab any physics object that you can carry with USE (I.E. A Pallet) and position it so that you can jump on it while holding it, and jump and continuously hammer the jump button. You should start flying into the air, and you can usualy reach max limit of the skybox before you drop the object (and thus, begin to fall.) Keep in mind its a difficult trick to master, and if you loose the object, you have the potential to be stuck! Save your game before trying this.