One night the captain of a tanker saw a light dead ahead. He directed his signalman to flash a signal to the light which went:
"Change course 10 degrees south."

The reply was quickly flashed back:
"You change course 10 degrees north."

The captain was a little annoyed at this reply and sent a further message:
"I am a captain. Change course 10 degrees south."

Back came the reply:
"I am an able-seaman. Change course 10 degrees north."

The captain was outraged at this reply and sent a message:
"I am a 240,000 tonne tanker. Change course 10 degrees south!"

Back came the reply:
"I am a lighthouse. Change course 10 degrees north."