Say vs. Tell

"I'm tired", I said. "What does my watch say? Isn't it after midnight? What do you have to say to that?"
"Now I'll say you goodnight.", he said.
"You can't tell the difference between 'say' and 'tell'", I told him.
"Is there a difference?", he said. "Of course!", I told him.

To the IT professional, the best way of explaining the difference between 'say' and 'tell' is that 'say' is SOURCE oriented and 'tell' is SINK oriented. Words say and said are usually used when the focus is on the speaker. Its taken for granted that there is someone hearing the speech. Sometimes the audience is implied from the context. Sometimes the presence/absence/person of the audience is not as important as the speaker. The words tell and told are used when the focus is on the audience. The audience is explicitly mentioned.

If you still have difficulty telling which one goes where, try not using the verbs at all. Let me re-phrase the above conversation:
"I'm tired", I exclaimed. "What's the time? Isn't it after midnight? What do you think we should do?"
"I think we should bid each other goodnight", he suggested.
"You can't differentiate between 'say' and 'tell'", I announced.
"Is there a difference?", he asked. "Of course!", I answered.

I think I will say my prayers now and wish you good-night.

Check out this link for more confusion:
http://www.learnenglish.org.uk/grammar/archive/sayandtell01.html

Check out how accurate you are here:
http://unamsanantoniolab.com/eng/troublesomeverbs/saytell.htm