Why is a tripod made with three legs instead of four? Explain your reasoning. Name some other objects that wor?

Aside from the quadripod issue (KMcG snuck in and stole my smart a$$ answer). a three-legged structure has the least number of legs necessary for basic stability. Any additional legs will just increase cost, complexity, weight, set-up time, and tripping hazards, which would get OSHA involved.