The riddle
What Breaks but Never Falls?
The pair
This riddle is usually asked in both directions. What breaks but never falls is daybreak. What falls but never breaks is nightfall. The two belong together: day breaks, night falls, and neither word works for the other. Merriam-Webster defines daybreak as the first appearance of light in the morning, and nightfall as the coming of night. Hearing both at once makes each easier to remember.
Verbs that go with the time of day
English pairs break with dawn and fall with dusk. Dawn breaks as the first light shows, and night falls as the light drops away. Merriam-Webster dates daybreak to 1530 and nightfall to 1700, which means the day word is about 170 years older.
How dawn is measured
The United States Naval Observatory defines the start of morning civil twilight as the moment the sun's center is 6 degrees below the horizon, and the same angle marks the end of evening twilight. Daybreak in the ordinary sense is a looser idea, but it begins near that moment, when the sky is bright enough to see without lamps. Measured that way, daybreak arrives before sunrise, while the sun is still below the horizon, and evening twilight ends the same 6 degrees below it after sunset.
A riddle to ask twice
Ask the first form, wait for guesses, then ask the second form. Groups often get the second faster once they have the first. Wrong guesses are easy to test against "never falls". A wave breaks too, but it falls back down when it ends, while daybreak has no falling part.