system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does smoking cause cancer?
gpt
So first: what causes cancer?  Our cells have an instruction booklet that's driven by the steps encoded in our DNA, a very complex molecule made of a huge list of simple "words" that together tell the cell what function to take, when to reproduce, when to do nothing, and when to die. 

When something bad happens to that book, those instructions can get garbled. So free floating energy, like a high-energy particle from the sun or from radiation, and certain chemicals that like to react with other stuff including your DNA, called free radicals, can sometimes come in and knock out a step in that manual. 

And if it happens to disable the step that says "Only reproduce so much and then stop" so that cell and all of its children start reproducing like crazy... well, that's cancer. And if some of that cancer happens to break off and travels throughout the body to be delivered somewhere else, well, that's really bad news indeed.

Smoking's incomplete combustion and delivery of all of those naughty chemicals in tobacco introduces a fair number of free radicals into the body. They get into our lungs and increase the odds that a DNA strand somewhere will have its "and then stop reproducing" switch turned off.