"Necessity is the mother of invention" means that inventions happen when there is a need for that invention. For instance, Samuel Morse created a code which made it easier to contact people, and he did this because his mother had him run messages to his neighbors on a horse. Morse code made this much easier. The cotton gin was invented to pick cotton from fields, as it was very hard labor. The cotton gin did the work of a hundred men, which made much less work for everyone. The telephone was also invented out of a need for communication. Alexander Graham Bell created the telephone and it made it very easy for people to contact one another, without actually having to meet them in person. These are some reasons why necessity is the mother of invention.

Image taken from (http://members.aol.com/SHRLIST/mother.jpg)