The design cycle
Design is a loop: understand the problem and who it is for, build, test, then use what failed to improve it.
A worked example
A paper bridge collapses when the fourth mass is hung on it. You look at where it tore, add material there, and test the new version the same way.
The mistake people actually make
Testing is for finding faults, not for proving you were right. A prototype is built cheaply and early precisely so a failure costs almost nothing.