system
Given the following post, respond as if you're commenting on Reddit:
human
why doesn't an atomic bomb cause a chain reaction and start spitting and exploding other atoms around it?
gpt
The same reason, more or less, that your fire doesn't burn the fireplace: the fireplace isn't made of fuel.

Only a few types of heavy atom are subject to neutron-induced fission. When neutrons hit (say) concrete, the concrete doesn't emit yet more neutrons. 

Thermonuclear detonations (fusion bombs) are even more analogous to a very high temperature fire: they work by heating light elements very hot, and those elements fuse and release more heat that allows the reaction to continue.  Most light elements can yield energy through fusion, so in principle a large enough hydrogen bomb could fuse (say) all the nitrogen in the air.  But it turns out to be very hard to keep the heat  from fusion concentrated enough to enable further fusion, just like a firecracker going off in a forest doesn't necessarily set all the trees on fire.  Except worse, because fusion requires very high temperatures to achieve and it takes something the size of a star to keep the "fire" going.