Know the difference between theory and reality

In theory, when you send an invoice, your customer pays on-time.

In reality, when you send an invoice, your customer sometimes pays, and seldomly on-time.

This rule applies to almost everything you learn at school/university and in textbooks.

Theory is useful, but it’s not real.

Only reality is real.

If you don’t know the difference between theory and reality, you will quickly go out of business.