The most accurate answer to what goes up and never comes down is a person's age. Another common answer is a person's height. Heat and smoke are also accurate answers for what goes up and never comes down, due to the fact that heat rises.
What goes up and never comes down is a popular riddle, as gravity causes practically everything in the universe to rise and fall. One of the most common wrong answers to this riddle is the sun, as it is a common belief that the sun comes up every day. The sun actually does not move, it is the Earth that rotates around the sun, giving the appearance of a rising sun. Helium balloons are also often perceived as going up and never coming down, but the helium reaches a certain point in the atmosphere and begins to dissipate, in turn, causing the balloon to descend back to the ground slowly.
Another common wrong answer to this riddle is gas. While gases definitely rise, they reach a limit in space and do not continue upward. The same goes for moisture. Humorous answers to this popular riddle include a cow walking up the stairs, inflation and gas prices.