The word "报警器" is formed from three characters: 报, meaning "to report"; 警, meaning "alert" or "police"; and 器, meaning "device". The combination of 报 and 警 creates the verb phrase "报警", meaning "to report an alert" or "to sound an alarm", and adding 器 specifies it as a physical device that performs this action, resulting in a noun for an alarm instrument like a burglar or fire alarm.