Some social-media posts claim that travelers roll a bottle underneath the bed to make sure no person is hiding there.

The basic logic is easy to understand.

If an object rolls underneath the bed and comes out the other side, the space appears to be unobstructed.

If the bottle stops, something could be blocking its path.

But there is an important limitation:

A water bottle is not a reliable security device.

It cannot prove that nobody is present.

Someone could be positioned in an area the bottle doesn’t reach. Furniture could block its path. The bed frame could have compartments or structural supports. The floor may be uneven. The bottle could simply get caught on a carpet edge.

So don’t treat the trick as a substitute for a proper safety check.

5. A Flashlight Is Still More Useful

If you’re genuinely concerned about what’s underneath a hotel bed, your smartphone flashlight is far more informative than a water bottle.