Classify 'X' as either a mammal, a bird, or an amphibian.
X: Frog
X: Frog is an amphibian.
Complete the following task based on the instruction and optional input provided.### Instruction:
Classify 'X' as either a mammal, a bird, or an amphibian.

### Input:
X: Frog

### Output:
X: Frog is an amphibian.