I'm not at home. Install the alarm. Someone will text me when they open the door. What's that thing called?

What you said belongs to the category of smart home. If it is realized, it needs a GSM module (short message module) and an infrared pyroelectric sensor. When a thief enters the house, the infrared pyroelectric sensor will give an alarm and output a corresponding signal to the GSM module, and the GSM module will send a short message "Someone has entered the house" to your mobile phone. If you make it more upscale, you can also add a camera to your home, and you can see real-time images of your home through your Android phone. This is very exciting.