system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why can we visibly see steam, but can’t see air normally outside? And why is it sometimes we can see the shadow of exhaust (like from a bus) but not exhaust itself?
gpt
When you can see steam, what you’re seeing is tiny drops of water being carried upward by the invisible water vapor. Since the air around steam is usually cooler than the steam, as soon as it evaporates tiny drops of water will condense out of the steam. It’s basically a cloud being carried up by hot air. 

The air outside usually has much less water vapor in it relative to its temperature. However, if there is a lot of water vapor in the air and it is a cold day, you will see the water vapor condense into fog. That’s also tiny droplets of water. 

As to the shadows, those happen because exhaust is much hotter than the surrounding air. This heat means that light travels at a different speed through the exhaust, and when light moves from a medium where it travels at one speed to a medium where it travels at a different speed, it bends, or refracts. You see the effect of that refraction as a shadow on the ground. For a similar effect, try holding a pair of glasses a short distance above the ground on a sunny day. They bend the light, creating a shadow.