If you've had your phone for more than a few days, you've probably noticed that occasionally it gets hot when you're using it. This is (almost always) normal. Here's why it happens.
Inside your phone is a processor very similar to the one in a computer. And like with a computer, when your phone's processor works it generates heat. The harder it works, the more heat it generates. This is just a side effect of all the crazy atomic electrical stuff going on inside it.
Desktops and laptops use fans, heatsinks, or even water systems to pull the heat away from all the important electrical chips---both to keep them safe and to keep them running---but phones don't have the space for such extreme cooling gear. Can you imagine your iPhone if it had a computer fan built in?
Since most methods of active cooling are off the table, phone manufacturers have had to come up with other ways to manage heat. The most basic is that the chips are just designed from the ground up not to generate that much heat. Unless you're putting your phone through its paces for an extended period---by doing something like gaming or video editing---it shouldn't run hot enough for you to feel it.
As phones have become more powerful, manufacturers have had to explore other ways of managing heat. Samsung has made headlines recently by bringing "water cooling" to their latest phones. Other phones use similar systems.
Now, so far I've been talking about the normal reason your phone gets hot, but it can also be a symptom of something more serious to do with the battery. If your phone is hot and you notice the battery start to swell, turn it off and contact the manufacturer immediately.
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