The riddle
What Flies Without Wings and Cries Without Eyes?
Reading the two clues
A cloud drifts across the sky and has no wings, so it flies in the loosest sense of the word. It has no eyes, but it lets rain fall, and rain is what the riddle calls crying. The question is searched in the first person as "i fly without wings, i cry without eyes", and the answer is a cloud either way. Some longer tellings add a line about darkness, which a cloud also brings when it covers the sun.
What a cloud is
Wikipedia defines a cloud as a visible mass of miniature liquid droplets, ice crystals or other particles suspended in the atmosphere. Most of the droplets and crystals are water. The U.S. Geological Survey's Water Science School describes how sunshine turns liquid water into vapor, which rises, cools into clouds and falls again as precipitation, and that last step is the crying.
A word that started as a hill
The Old English words clud and clod meant a hill or a mass of stone. Around the early 13th century, according to the same article, the word began to describe rain clouds, because they resembled heaps of rock in the sky. It gradually replaced the Old English weolcan, which had meant clouds in general. A word for a mass of stone now names something that floats.
What the droplets form on
Wikipedia adds that condensation usually happens on tiny particles such as salt or dust that are small enough to be held up by the normal circulation of the air. For another riddle that describes something by what it does, see the one about what falls but cannot rise.