Dragons, in truth, are much stronger than stories tell. A knight with just a steel sword and armor could never slay a Dragon more than his own size. The true fact is the knight would be cooked inside his armor as soon as the Dragon spit fire out. Water isn't a Dragons weakness either, It's a giant lizard, and lizards are an aquatic animal as well as a land animal. The Dragons scales are extremely thick and nearly impossible to penetrate, unless a harpoon was used. Some of the only ways to kill a Dragon is to explode it from the inside-out, shoot a large spear through it's chest, or sever it's head(also nearly impossible).
"A Dragon has the ability to breathe fire" is a common misconception. The Dragon doesn't have the ability to breathe fire, merely, it has the ability to spit a liquid out of it's mouth and heat it up very quickly so that it combusts and turns into fire. A Dragon can't live in lava or breathe fire in. They can live in very hot places, such as a Volcano, but if they touch lava they melt/burst into flames just like everybody else. Breathing in fire would also hurt them, because although the outside can withstand fire, the inside of a Dragon is just like the inside of any other lizard, or any other animal for that matter.
A Dragon does have the ability to fly, but that requires energy, a Dragon is smart enough to know that the less energy it uses, the longer it can last while hnting for food. When a Dragon does take to the skies, it's usually leaving or it's about to use fire to cause wide-spread damage and chaos, but it only does this if it thinks it's got a good chance of killing many instead of just a few like it would on the ground. A Dragon's intelligence depends on what type of Dragon it is, like Wyvern or Wyrm
To learn more about Dragon Species or see some Dragon pics, click one of the links below!