system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do you have to pour acids into water and not the other way around?

Why do you have to pour acids into water and not the other way around? And does it change if doing two different acids, or a base and water?
gpt
When concentrated acids become dilute, they release a lot of heat. 

This heat can and will boil water, causing it to splash/explode out of the flask.

Heat increases temperature less the more mass there is. Furthermore, the less acid is diluted at once, the less heat is released.


So!

You have a pool of concentrated acid. You add a tiny bit of water. A lot of heat is released where the water impacts, gets transferred to the water. Then, the water boils and causes a splash.

On the other hand, you have a pool of water. You add a tiny bit of acid. Heat is released, but far less. You swish and swirl the flask, distributing the heat across the whole liquid rather than where acid enters the water. By doing so, no point of water can get hot enough to boil. 

It can still end up overheating - if you're diluting a lot of acid. In such cases, you either put the flask in a ice-water bath or just wait for it to cool back down. This is especially likely when dissolving drain cleaner (sodium hydroxide - a base, but similar mechanism).