Surprise an intruder

What can we use instead of the small bell or paper cup if those are hard to find?

If you don't have a small bell, tie a metal washer, a bottle cap threaded with beads, or a few jingle charms to the string, and if you don't have a paper cup use a small yogurt container or a folded cardboard cup in the step where you make the hole and tape it to the door frame.

Why doesn't the bell ring or the cup fall off when I test it, and how do I fix that?

If the bell doesn't ring or the cup falls, make sure the hole is centered in the bottom of the cup, retie a bigger knot on the inside so the string can't pull through, shorten the string so the bell hangs closer to the doorknob, and use stronger tape or mounting strips to secure the cup to the top edge of the door frame as instructed.

How can I adapt this door alarm for different ages or skill levels?

For preschoolers have an adult make the hole and use thick yarn and wide tape while they decorate the cup, for elementary kids let them cut, tie and adjust the string with supervision, and for older kids let them design adjustable lengths, add multiple bells, and handle the full taping and testing steps themselves.

How can we make the door alarm louder, more secure, or more personal after finishing the basic version?

To enhance it, add extra bells or beads to the string for a louder jingle, reinforce the cup to the frame with stronger adhesive tape or a small screw (with adult help), and personalize by decorating the cup with markers, stickers, or paint before testing and sharing on DIY.org.