It is big. It is long. It is silver. It is fast. It has many doors. What is it?
It is the MTR.
It is small. It is red, blue or green. It has four wheels. It can carry 4 to 5 people. It is expensive. What is it?
It is a taxi.
It is small. It is noisy. It flies in the sky. What is it?
It is a helicopter.