Answer:
Five months have less than 31 days. They are February (28 days), April (30 days), June (30 days), September (30 days), and November (30 days).
Explanation:
There are a couple of tricks to do this, but I have the length of each month memorized. One trick if the knuckle trick. In this tactic, you start from the knuckle on your pinkie and count the space between your knuckles in a knuckle-skin-knuckle-skin pattern. You hit the knuckle by your thumb two times in this pattern and you won't make it all the way back to the knuckle by your pinkie. List the months in order while you are doing this motion. Every time you hit a knuckle while saying a month, it is a month with 31 days. Every time you hit the skin, it is a month with less than 31 days. If you need a visual for this motion, please use google to help (because they won't let me post a link :P)
I hope this helped!