Here's why:
* Simile: A simile uses the words "like" or "as" to compare two things.
* Metaphor: A metaphor directly states that one thing *is* another thing, without using "like" or "as."
The sentence "Food without salt is like eating cardboard" uses the word "like," making it a simile. It compares the experience of eating unsalted food to the experience of eating cardboard.