Here's why:
* Hair length is consistent: Hair grows from the scalp at a consistent rate. So, if a boy has short hair, it means his hair hasn't grown long enough to reach his waist.
* Growth takes time: It takes time for hair to grow long. Even if the boy starts growing his hair, it will take a while to reach his waist.
Perhaps the question is trying to make you think about:
* Hair extensions: Some people use hair extensions to make their hair look longer, but this is temporary.
* A change over time: The boy could have had short hair in the past and is now growing it out to waist length.
Let me know if you'd like to explore any of those ideas further!