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admonish	to warn or caution. [1/2 definitions]
alert	to warn or make aware. [1/3 definitions]
fire alarm	A fire alarm is a device that makes a loud noise to warn people that there is a fire.
had better	When we say "you had better" or "they had better" we want to warn someone that something bad might happen if they don't do something. If your mom says you had better be careful when crossing the street, she means that you really need to be careful. She doesn't want anything bad to happen to you.
lecture	a long speech given to warn or scold another. [2/4 definitions]
notice	A notice is a sign or something printed in a newspaper or other place to tell you or warn you about something. [1/4 definitions]
ozone hole	an area in the ozone layer where the ozone becomes thin. There is a large ozone hole over Antarctica and a smaller one over the North Pole. Scientists warn us this is dangerous.
smoke detector	a device that makes a loud noise to warn people of smoke or fire.
threaten	to warn or give a sign of. [1/4 definitions]
timber	"Look out!" (used by loggers to warn of a falling tree). [1/4 definitions]
warn	When you warn someone, you tell them about something that might be dangerous or hurt them. [2 definitions]

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