If you need a quick alarm to alert you to intruders, look no further than some 4th of July poppers and some duct tape. Here's how it works:
1) Tape the bottom part of the popper to the door.
2) Tape the string of the popper to the door frame.
3) Wait for someone to open the door; when the door is opened, the string will be pulled away and set off the popper, making a loud enough noise to alert you to the intruder.