Why Don't Fire Hydrants Freeze and Explode?

Pipes can freeze in the winter, but you never hear about fire hydrants freezing. What makes them safe from the cold temperatures in winter time?

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Sources:

http://www.todayifoundout.com/index.p...
http://media.cygnus.com/files/base/FH...
http://media.cygnus.com/files/base/FH...
https://www.livescience.com/43408-why...
https://www.plumbingsupply.com/reside...
http://www.firehydrant.org/info/faqs_...
https://books.google.com/books?id=GUX...

Images:

https://en.wikipedia.org/wiki/File:Do...
https://en.wikipedia.org/wiki/File:Dr...
https://commons.wikimedia.org/wiki/Fi...
https://commons.wikimedia.org/wiki/Fi...
https://commons.wikimedia.org/wiki/Fi...
https://en.wikipedia.org/wiki/File:18...
https://commons.wikimedia.org/wiki/Fi...