The riddle
What Does a Snowman Eat for Breakfast?
Frost and flakes
Frosted Flakes is the name of a breakfast cereal, and the name itself is the joke. Frost is the ice that coats cold surfaces, and flakes calls snow to mind. A snowman made of snow could eat nothing better suited, so the answer sounds like a menu planned for him.
The other answer
Many tellers say snowflakes instead, which makes a different kind of sense: a snowman would eat what he is made of. Both answers are accepted. Frosted Flakes depends on knowing the cereal, so it works best with listeners who have seen it on a shelf.
What a snowflake is
A snow crystal is a tiny piece of ice that grows in a cloud, and most have six sides or six arms. A snowflake can be a single crystal or many crystals stuck together, according to the National Snow and Ice Data Center. Kenneth Libbrecht's book The Snowflake (2003) shows how many shapes they take.
What frost is
Frost is ice that forms on a cold surface when water vapor in the air turns straight into ice, according to the National Weather Service. It needs a surface at or below the freezing point of water, 32 degrees Fahrenheit or 0 degrees Celsius. That makes "frosted" a good word for a snowman's cereal. Ask the riddle on a snowy morning and the listeners may be looking at a frosted window.