"You don't say" depending on how and when it is spoken can have a few different meanings. In this context, it could mean "really?" or "is that right?". "You don't say" could also be used to tell someone to not speak a certain way or use specific vocabulary. Example 1: John: I haven't slept in 2 days! Sue: You don't say? John: Really, I haven't gone to sleep in 2 full days. Example 2: John: Tonight I am going to count lamb until I fall asleep. Sue: You don't say it like that. You say, I am going to count sheep until I fall asleep. "You tell me" also could mean different things depending on context but it is basically Subj 2 asking (or telling) subject 1 to vocalize something. Example 1 : John: Do I look like I haven't slept in 2 days? Sue: [holds up a mirror] I don't know. You tell me. John: Yeah. I look tired. Example 2: Sue: Ok. Let's go home. You need to sleep. John: But, I want to talk for another hour. Sue: I'm confused. First you tell me you are tired and now you tell me that you want to talk. John: I'm sorry. Ok. Let's go home. I hope this helps!

In most cases, "You don't say" is used as a sarcastic way of expression. Person A: I am wearing a watch Person B: You don't say! (wow really? I cannot believe you are wearing a watch :O ) "You tell me" is something you would say when you're ordering some one to tell you something. (It's a little aggressive) Person A: What are you doing here? Person B: You tell me, what does it look like I'm doing here. OR When you suspect some one knows something but they're hiding it. (Some one who is prone to being suspicious) Stranger: Who do you think killed Lorenzo? Police man: I don't know, you tell me >.>

'You don't say' is often used as an expression to express sarcasm about what someone has said. Eg. A lazy teenager complains that he had to wash all the dishes by himself. You could respond: 'You don't say' - meaning 'You have nothing to complain about'. 'You tell me' is used when someone asks a question that you think they should already know the answer to. Our lazy teenager asks 'Where is the dishwash detergent?' You might reply 'You tell me' - meaning 'You should already know where the detergent is kept.' It is commonly used by teachers of course. A child asks, 'How do I spell 'rhythm'?' Teacher responds 'You tell me'.