You are absolutely right! Water vapor is one of the constituents in air.

Here's why:

* Air is a mixture: Air is not a single substance, but a mixture of gases.

* Water vapor is a gas: When water evaporates, it becomes water vapor, which is a gaseous form of water.

* Variable amounts: The amount of water vapor in the air varies depending on factors like temperature, humidity, and location.

So, while not always the most abundant component, water vapor is definitely a significant part of the air we breathe.