A puddle disappears because its water moves somewhere else.
Evaporation is usually the biggest visible reason.
Evaporation changes liquid into vapor
Water molecules in a puddle are always moving.
Some surface molecules have enough energy to escape into the air as invisible water vapor.
The water still exists.
It is simply spread through the atmosphere.
Does water need sunlight to evaporate?
No.
Warmth usually speeds evaporation, and sunlight can warm the puddle.
But water evaporates on cool or cloudy days too.
Boiling is not required.
Wind makes a difference
Air directly above a puddle can become humid.
Wind replaces that humid air with drier air.
That usually allows evaporation to continue faster.
Humidity slows evaporation
If the air already contains a lot of water vapor, it is harder for more water to escape from the puddle.
That is why damp weather can keep wet surfaces wet longer.
Some water goes into the ground
On soil, grass, gravel, or cracked pavement, part of the puddle may soak downward.
Drainage can also move water somewhere else.
Where does evaporated water go?
Eventually it may become part of clouds, fog, dew, rain, or snow.
That links the humble puddle to larger processes such as how fog forms.
The water did not vanish.
It changed form or moved somewhere you cannot see.
And that’s the short explanation.
Related explanations: Why does fog form?, Why do clouds stay in the sky?, and Why can you see your breath in cold weather?