Find and cite the metaphor used in the passage.
Life is like a roller coaster; it has its ups and downs, and you never know what lies ahead.
The metaphor used in the passage is: "Life is like a roller coaster."
Complete the following task based on the instruction and optional input provided.### Instruction:
Find and cite the metaphor used in the passage.

### Input:
Life is like a roller coaster; it has its ups and downs, and you never know what lies ahead.

### Output:
The metaphor used in the passage is: "Life is like a roller coaster."