Here's how to tell the difference:
* "Like" is used when comparing two things that are not the same.
* Example: "The sky is like a canvas painted blue." (The sky isn't actually a canvas)
* "As" is used when comparing two things that are similar in some way.
* Example: "He ran as fast as a cheetah." (Both the person and a cheetah are capable of running fast)
It's important to note that similes are not metaphors. A metaphor states that one thing is another thing, while a simile compares two things.