Many psychology experiments have shown that our short-term memory can hold only a limited number of separate items. The average is about 7 items, plus or minus 2, depending on the individual. You can easily confirm this by reading someone a series of words that have no connection with one another, then asking this person to repeat them. Start with 1 word, then 2 different words, then 3 different words, and so on. When you get up to about 7 words, chances are the person will start making mistakes!

The following is another easy test that demonstrates the volatility of short-term memory. Read a telephone number to someone out loud. Wait 10 seconds; then ask them to repeat it. They should be able to do so without any problem. Now read them another telephone number, but this time ask them to count down from 100 by threes (100, 97, 94, 91, etc.) for 10 seconds, then repeat the phone number. By the time they finish counting down, they will probably have forgotten it.

The simple distraction of having to perform a few subtractions in their head prevented them from mentally repeating the telephone number to keep refreshing their short-term memory. Short-term memory, also known as “working memory”, cannot retain information for more than a few seconds. To remember something for any longer, you must therefore repeat it to yourself several times, long enough to modify your long-term memory.