Sort the following list of words into two categories: verbs and nouns.
walk, tree, cat, river
Verbs: walk

Nouns: tree, cat, river.
Complete the following task based on the instruction and optional input provided.### Instruction:
Sort the following list of words into two categories: verbs and nouns.

### Input:
walk, tree, cat, river

### Output:
Verbs: walk

Nouns: tree, cat, river.