What does it mean when a person puts a glass of water with a saucer on top, by their bed side at night?

Question

Or a full glass of water on the window seal.

Answer 1

That's for when they get thirsty in the middle of the night.

Answer 2

It traps the spirit in the water.

Answer 3

So that nothing falls in the water

Answer 4

They don't want the cat coming along and sneaking drinks from their glass of water.