Why do triangular shapes work best for distributing weight in bridge building?

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HeJind
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Sorry if my English is bad. We're building bridges and learning about truss and truss variations, but I'm still confused as to why the triangular shapes work best and what they achieve. How would I know side would be compressed and which would be tense?

We have to build a bridge using roughly 10 dozen popsicle sticks. If I want to apply pressure/load to the middle, how would I design the bridge to distribute this evenly?

Apparantly last year someone got over 570llbs
 
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Triangles "work best" because they're the only convex polygon that has intrinsic strength. You cannot change any angle of a triangle without also changing the length of one of the sides. You can change the angles of any other convex polygon without changing the length of any side; e.g. a square can be deformed into a parallelogram.