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This one is a bit tricky! Once you get the gravity gun, you'll subsequently meet DOG. DOG's ball is inside a container under a large rock. DOG will eventually remove the rock, and the container will start moving. Remove the container to find the ball inside. Make a quicksave here!

Close to where Alyx is sitting, there is a basketball hoop. To get this achievement, you must shoot DOG's ball into the hoop. This is easier said than done, because you have a time limit, and the gravity gun fires hard!

So position yourself quite a bit back, lining up between a wooden pallet on your right and a car on your left. Aim so that the hoop is visible just below the ball(not counting its protrusions, which might get in the way of aiming) as you're holding it, then fire! You might not get it on the first try, but using this rough technique you should be able to get it rather quickly!

If you don't get it before the attack, just quickload.