Here's why:

* Invisible: Hydrogen gas is a very small molecule, and it doesn't interact with light in a way that makes it visible to the human eye.

* Odorless: Pure hydrogen gas has no smell.

However, you might sometimes see or smell hydrogen gas indirectly:

* Burning hydrogen: When hydrogen gas burns, it produces a pale blue flame.

* Hydrogen leaks: If there's a leak of hydrogen gas, you might see bubbles forming in water or other liquids, or you might smell a faint odor if the gas is mixed with other compounds.

Let me know if you have any other questions!