When you use the same things, everything looks similar. Dirac became quite a legend for taking the best square root in town. Lagrange must have invented the Lagrangian, because that's his name. That's too sophisticated. We'll do simple field theory, where everything I say is correct.
Many people find physics difficult.
It must be momentum. I will especially use the fact that it is meaningless. When something has no meaning, you can do whatever you want. A derivation is not something you look forward to on Friday. Hopefully time will run out so I won't have to do it.