"My friend and I found it in the creek, it was full of cement. It has clearly been in there for a while, and we found it in California.
A: "That is a ASTM C31 / C31M Concrete testing cylinder. You fill them with concrete, let it harden overnight in a cure box, then place them in a limewater bath for 7,14,28 days (depending on the test requirements) to do strength testing of poured concrete."
This. Learning to sharpen your knife by hand is not easy at first, but it's worth it.
It detects the presence of an electric charge.