As I explained in Rhythm in Poetry – The Basics, some syllables in English are “stressed” – pronounced louder or with more emphasis than others – while other syllables are “unstressed,” meaning they are not emphasized. Knowing this, you can create patterns of stressed and unstressed syllables in your writing to create a rhythm in the words. Having rhythms in your poems make them more fun to recite and easier to remember.
To make it easy to spot the stressed and unstressed syllables in the examples I gave, I wrote them in UPPERCASE and lowercase letters, like this:
my PUPpy PUNCHED me IN the EYE.
The trouble with using this method is that it is awkward to write or type this way, and it makes the poem more difficult to read. Also, if you have a poem that is already printed on paper, you wouldn’t want to have to rewrite the entire thing just to show the rhythm.
Wouldn’t it be better if could make marks to show the stressed and unstressed syllables? Indeed, there is such a system that is commonly used, and it’s called “scansion” (pronounced “scan-shun”). The process of marking the stressed and unstressed syllables in a poem is called “scanning.”