The riddle
What Is at the End of a Rainbow?
Read the word
Rainbow is spelled r, a, i, n, b, o, w, and the last letter is W. The question seems to ask about the sky, and listeners prepare answers about faraway places. The riddle asks about the word instead. Anything at the end of the word rainbow is the letter w. The listener who thinks of the sky has been shown a landscape when the answer was printed in the question all along.
What the sky says
The National Weather Service explains that a rainbow is really circular, but the ground blocks the lower part, so people see an arc. A rainbow forms when sunlight enters a raindrop, reflects off the inside back wall and leaves at an angle of 42 degrees. If the sun is more than 42 degrees above the horizon, no rainbow is visible, and the lower the sun, the more of the circle shows.
Why there is nothing to reach
Since the bow is made by light bending in drops, it depends on where the viewer stands relative to the sun and the rain. Move sideways and the rainbow moves too (Britannica, "rainbow"). It is a pattern of light seen from the viewer's own position, not an object sitting in a field, so a traveler can never come to the end of one.
The word bow
In rainbow, bow means something bent into a curve or arch (Merriam-Webster, "bow"). Rain supplies the other half. Together they say a curve made by rain, which is a literal description. A group asked to find the letter at the end of other weather words, such as thunderstorm or snowflake, will land on m and e.