But it can also be used to warn someone that the water has been poisoned.

The expression is usually used to warn against risky action.

The signs are used to warn drivers of people walking in the street.

These signs are used to warn of level crossings ahead.

Today it is used to warn players of an imminent danger or disagreement in the game.

But these lights are used as a simple code to warn a criminal.

Radio was used to warn shore stations and ships of danger.

The beacon was used to warn the residents of an invasion.

A bell may be used to warn of a cyclist's presence.

It had historically been used as a beacon to warn of invasion.