In Braid 3-1, there's a locked door on the opposite side of a large pit. At the bottom of the pit there's a goomba carrying the key to the door. I've managed to time it so that when I jump down onto the goomba, I pick up the key and bounce back up - but I'm not bouncing up high enough to escape the pit. What's the trick?
Each area in Braid introduces a special mechanism. The first level of each area, The Pit, allows you to experiment with the mechanism, so I would advice you to play around a bit first. That said, the special mechanism for the third region is:
Objects that have a green glow around them are not affected by your time reversal ability
Therefore, the solution to the problem is to:
Go down, kill the Goomba and get the key - note its green glow - and reverse time so that you get back to where you started. They key will remain in your hands, and you can then use it to open the door.
Here's a YouTube video walkthough if you need any more help.