From the collection Written Proficiency Comments, by A. Kostia

You cannot write an essay

just by taking sentences

from other sources and replacing

one or two words in them. Sources

are for information, not for

copying sentences from. Write your own

text. Use your own

words. Write about why you believe

these inventions are so significant. Focus

on their role rather than on

random facts about their history.

This should be your own

essay, not a rehash

of a Wikipedia entry.