The answer to the riddle "What gets wetter the longer it’s in the sun?" is a towel. As a towel dries something off, like your body after a swim, it absorbs water and becomes wetter itself. The more it is used in the sun, drying off wet surfaces, the wetter it becomes.
a towel
Towels
A towel.
An umbrella gets wetter but it keeps you dry.
Towels
A towel gets wetter and wetter as it dries.
a towel
Towels
A towel.
An umbrella gets wetter but it keeps you dry.
A towel gets wetter as it dries the person using it.
Towels
a towel!
the atmosphere.
a towel
A towel.
TOWEL