Collect some muddy water from a puddle or a pond. Hold the can over a bowl and pour the muddy water into the can. Look at the water that comes out of the bottom of the can into the bowl. It is much cleaner than the water you poured in!
The water may look good enough to drink, but you should not drink it. It may still contain harmful bacteria that your filter did not remove.
Go back to the Water in Chicago's Community page.