We decided we'd test the bridge with sand in a bucket. We bought two 60 lb bags of sand, thinking surely my bridge would break under 120 lbs. It held 120 lbs. We emptied the sand out, and added 40 lbs of water, then added all the sand back. It still held.
We tried the intern, and it still held.
Two days later we bought another 150 lbs of sand. The bridge finally broke under 205 lbs!