You can only remember 3-4 things

Alan Baddeley, a British psychologist, did many studies on working memory, and concluded that you can remember about 3-4 things (for about 20 seconds) and then they will disappear from memory unless you repeat them over and over.

For example, let’s say you are driving in your car and talking on your cell phone and someone gives you a number to call. You don’t have a pen handy, so you try to memorize the number long enough to hang up from one call and dial the new number. What do you do? You repeat the number over and over (putting it back into short term memory each time, which buys you another 20 seconds). The thing about phone numbers is that they are more than 3 or 4 numbers long. So they are hard to remember for more than 20 seconds.