Can you help me to categorize these things as either "expensive", "not expensive", or "I don't know": air, gold, rock, rocket and dream.
Air and rock are not expensive, but gold and rocket are expensive. I don't know if dream is expensive or not.
Below is an instruction that describes a task, paired with an input that provides further context. Write a response that appropriately completes the request.
### Instruction: Can you help me to categorize these things as either "expensive", "not expensive", or "I don't know": air, gold, rock, rocket and dream.
### Input: 
### Response: Air and rock are not expensive, but gold and rocket are expensive. I don't know if dream is expensive or not.