" Some of the test subjects were given cards that had both words printed in full, like this:

Hot: Cold

Others used cards that showed only the first letter of the second word, like this:

Hot: C

The people who used the cards with the missing letters performed much better in a subsequent test measuring how well they remembered the word pairs. Simply forcing their minds to fill in a blank, to act rather than observe, led to stronger retention of information. "

― Nicholas Carr , The Glass Cage: How Our Computers Are Changing Us