Last week my physics class studied solids. I found the material in the book to be a bit thin & lacking, so I tied in a unit on bridges, and then had them build toothpick bridges. They could only use glue to connect the toothpicks - and even then, they could only use it at connecting points (no coating the toothpicks in glue, etc.). They were graded on a few things, one of which was how much mass it could support.
Here was our first candidate:
Looks solid, but when testing begin.... not so much! We tested the bridges to failure - meaning when they broke or flexed more than 2cm. This one lasted about 1200g - only about 2.6 pounds.
Candidate #2 - they realized quickly that using triangles was much sturdier than anything flat. I loved that they did horizontal ties to simulate a roadway. It was also an incredibly light bridge.
In the end, it supported about 4400g (9.7 pounds) before it snapped.
Candidate #3 I nicknamed the Beast. EVERYTHING was triangles - including each individual beam (3 toothpicks glued together). It supported massive quantities of weight...
.... almost 13000g (close to 29 pounds!!) before it broke!