What’s the essential difference between the two examples given before? (bridge vs. superconductor
When we build a bridge,
1. It’s impractical to build a bridge many times.
2. It’s not necessary to build many bridges.

When we build a superconductor, we need to attempt many times because
1. Superconductors serve as the building blocks of other materials, it’s necessary to make the superconductor really work.
2. It’s practical to do it many times.