Something is cool if it is comfortably cold. The picture is of a cool drink. Something that is popular, particularly with young people, can also be described as cool.
Drool is another word for saliva, especially when it is dripping from your mouth. You can often see dogs and babies drooling.
A duel is a fight between two people that are both using the same or similar weapons. Duels were once a formal way of settling a disagreement or as payback for an insult. Duels often had rules of conduct, and duels happened between two people of equal social status, usually gentlemen or upper class men. A duel is not the same as dual.
A footstool or footrest is a small piece of furniture designed for you to rest your feet on, usually while you are relaxing on a sofa.
Fuel is something that can be turned into energy. Firewood, coal and gasoline are all fuels that can be burnt to produce fire and heat. Uranium is fuel in a nuclear reactor. Food is fuel for people.
Granules are small, rounded pieces of something. They can be made by grinding something down into smaller pieces or more commonly by building them up from clumping much smaller ones together. The granules in the picture are coffee granules.
A jewel is a cut and polished gemstone, either precious or semi-precious, that can be used to make jewelry. Another word for a jewel is a gem.
A mule is what you get when you cross a male donkey with a female horse. All male mules and most female mules are infertile. Mules make good working animals as they inherit many of the good traits from both donkeys and horses.