The riddle
What Comes Down but Never Goes Up?
True of the drops
A raindrop falls and does not climb back up as a drop. The water does return to the sky, but in another form. The USGS Water Science School describes how sunshine turns liquid water into vapor, which rises, cools into clouds and falls again as precipitation. The riddle is accurate about the rain itself and silent about the rest of the cycle.
A follow-up question
Once the answer is out, ask where the water goes next. According to the USGS, most precipitation falls straight back onto the oceans, and the rest lands on the ground, where some of it soaks in, some runs into streams and some evaporates to start the trip again.
Other answers that come down
Snow, hail and sleet also come down, and the USGS counts them with rain as precipitation. Any of them is a fair answer, though rain is the one most listeners reach for.
The mirror of another riddle
The question reads as the reverse of what goes up but never comes down. Asking the two back to back lets listeners notice that one has an answer in the sky and the other in a birthday.