In general, they are explained here:
http://en.wikipedia.org/wiki/Mnemonic
Here is a particular type where you count up the number of letters in each word to
get a digit. And this example also has to do with the meaning of what is desired
to remember (some digits of a number).
\(\displaystyle \text{I have a root of a two whose square is two.}\)
\(\displaystyle 1 \ \ \ 4 \ \ \ 1 \ \ \ 4 \ \ \ 2 \ \ 1 \ \ \ 3 \ \ \ \ \ \ 5 \ \ \ \ \ \ \ 6 \ \ \ \ \ 2 \ \ \ 3\)
\(\displaystyle \sqrt{2} \ = \ 1.414213562373...\)
http://en.wikipedia.org/wiki/Mnemonic
Here is a particular type where you count up the number of letters in each word to
get a digit. And this example also has to do with the meaning of what is desired
to remember (some digits of a number).
\(\displaystyle \text{I have a root of a two whose square is two.}\)
\(\displaystyle 1 \ \ \ 4 \ \ \ 1 \ \ \ 4 \ \ \ 2 \ \ 1 \ \ \ 3 \ \ \ \ \ \ 5 \ \ \ \ \ \ \ 6 \ \ \ \ \ 2 \ \ \ 3\)
\(\displaystyle \sqrt{2} \ = \ 1.414213562373...\)