Ask "WHY?" 5 Times

Often it is necessary to ask "why" several times to trace a problem back to its root cause. For example:

Q1. WHY did the machine stop?
A1. Because the fuse blew due to an overload.

Q2. WHY was there an overload?
A2. Because the bearing wasn't getting enough oil.

Q3. WHY wasn't the bearing getting enough oil?
A3. Because the oil pump wasn't working properly.

Q4. WHY wasn't the oil pump working properly?
A4. Because the pump shaft was worn.

Q5. WHY was the pump shaft worn?
A5. Because sludge got in.