That one is easy. Just at the beginning of your tour into the cave, take a postcard from the gift shop and go with it into the cave (after you gave the guy these three objects from the very first puzzle). The achievement pops right after you got through the door.
Note:If you want to get this achievement along with the following achievement:
The Remorse achievement in The Cave worth 11 pointsRemembered it's wrong to steal. you will have to transport this postcard over your whole journey in the cave to return it right at the end! This is not too hard, just a little frustrating because of the item swapping you'll have to do.