YSK: water heaters have an anode rod that prevents the tank from corroding. If you replace it every few years, it will extend the life of your water heater from ~10 years to potentially 25+ years.
from /r/YouShouldKnow
YSK：熱水器有一個陽極棒，可以防止水箱被腐蝕。如果你每隔幾年就更換一次，就可以將熱水器的壽命從~10年延長到可能的25年以上。