You are given a card so when you buy a cup of coffee you get closer each time until you get a free cup of coffee.

Card A- Has 10 boxes for stamps and when you reach to the 10th box you get a free cup of coffee.
Card B- Has 12 boxes for stamps with 2 already stamped and when you reach the 12th box you get a free cup of coffee.

Which card do you think will get filled up first, even though its the same amount of boxes that need to be stamped?

Card B will be filled up faster because of goal-gradient effect. This means it will accelerate your behavior as you progress closer to your goal.

This scenario was studied by Ran Kivetz in 2006 after Clark Hull in 1934 did a research on rats where he noticed they ran faster when they saw the food at the end of the maze. In addition to the coffee card scenario Kivetz did a similar study with a web site.