When recycling became popular in the United States in the early 1990s, American’s were instructed to sort their recyclables into separate bins. Plastic went into one bin, paper in another, glass sorted into another bin. By the end of the decade, technology that allowed single-stream recycling was developed. At last, even the laziest among us had no excuse to avoid recycling.

Under the single-stream recycling system, all recyclables go into a single bin. At the recycling plant, recyclables are loaded onto a conveyer belt and separated in response to certain stimuli: Magnets lift metals from the belt; puffs of air blow paper from the line, etc. 
Check out the video I posted in the Media section about how single stream recycling works at the waste management plant!