The riddle
What Do You Call an Old Snowman?
Snow, then water
Snowmen are built of snow, and the National Snow and Ice Data Center describes snow as ice crystals. Ice melts at 0 degrees Celsius, which is 32 degrees Fahrenheit, according to the United States Geological Survey. A snowman that has stood for days in mild weather has gone past that point and turned back into the water it came from. "Old" in the riddle means long enough for that to happen.
How much water is in snow
Snow is mostly air between the crystals. A common rule of thumb says 10 inches of fresh snow melt down to about 1 inch of water. That ratio is only a rough guide. A 2005 climatology of snow-to-liquid ratios across the contiguous United States, by Baxter and colleagues, found that it shifts with temperature and place. Wet snow near freezing is denser, so the same depth holds more water than light, dry snow from deep cold.
A puddle is another reply
Snowman riddles often end in "puddle" when the question is about a snowman in summer or with a suntan. This one is simpler. An old snowman is the water itself, which makes the answer shorter than most of its cousins.
Why the answer is not a joke word
Many riddles of this shape end in a pun. This one ends in a plain noun, and the humor lies in how obvious the answer is once someone says it. Listeners often search for a pun that is not there, and some groan when none turns up. A pause after the question gives them time to overthink it, and the answer lands harder.