1st rip a piece of duct tape about 5" long, then place the firecracker on it. make sure that there is a little sticking out. then fold over one of the corners over the fire cracker. Then tape it to the wall, after that reinforce the duct tape with multiple pieces of tape. Now tape a small piece of tape and wrap it around the fire cracker string. This will ensure a pull, instead of the string letting go and not making the firecracker go boom. Then tie the other string around the tape on the end of the firecracker.
Step 2: The other end
now take a 5" piece of tape and put it to the other side of the string. This piece will go on the inside of the wall. So now carefully close the door, but make sure your hand can fit in the door way then just tape the string to the inside of the door way. now carefully close the door and walk away, the next person to open the door will get a not - so- pleasant - surprise!!!