What exactly are "sight words" and why are they important for beginning readers to learn? "For instructional purposes these are usually referred to as sight words or sight vocabulary because we would like our students to recognize them in less than a second. Why? Because this enhances their chance of getting to the end of a sentence in time to remember how it began" (Frank B. May, "Reading as Communication").
How should we decide which words to teach as sight words? Studies suggest that the most in our language, known as "high-frequency words" be the best place to start. The 100 most common words actually make up about 50% of the material we read! The 25 most common words make up about 1/3 of our written material (Fry, Kress, and Fountoukidis, 2004). Furthermore, Many of these words do not sound like their spellings might suggest, so "sounding them out" would be unproductive.