First get a piece of string. 

Step 1:

 Then find 2 things to tie it to and put it across the door. Keep the string low.

Step 2:

 Then wait till somebody trips.
When the person comes to trip over, doesn't the table and the chair (for example) move?
 not if you have 2 good things to tie it to
<p>That's how you know someone has tripped over it!</p>

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More by milesBob:How to throw a playing card! How to make a trip wire! 
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