How can you start a fire using a hammer and an anvil
Metal gets hot when you deform it.
You can try this yourself, grab a paperclip and and bend it back and forth very rapidly for a bit, the spot where it was bent will get quite hot.
The guy in the video is deforming a metal rod by hammering it. Once it’s hot enough you can use it to ignite a fire
referring to the clip u/JerseyWiseguy linked, whenever energy is released, some of it always is converted to heat, the lowest form of energy.
As the hammer repeatedly strikes the equally hard anvil and the rod in between, some of that kinetic energy used to swing the hammer is being converted to heat on impact. Metal tends to conduct and retain heat very well, so that rod is gaining heat with each strike faster than it loses heat.
If you’re referring to the guy hitting a metal rod and then using it to ignite a peice of paper, 2 things:
1) Compression generates heat. Every time the rod was struck, it compressed slightly, which increased the temperature.
2) Even if there was no compression, in pretty much every non-perfect physical interaction, some energy is transferred into heat. Hit something hard, fast, and constantly enough and it will raise in temperature significantly. You can simulate this yourself; Rubbing your hands together is essentially just hitting them over and over, and it very quickly warms them up.
Is this a riddle? Not really the place for it.