Chinese people don't just use tones - they use intonation as well in much the same way we do...so in the class room, the phrase "Ming bai?" (are you clear?) can be answered with "Ming Bai." as a statement because you use different intonation patterens [sic].
I think I should try to explain a few things before I start...
A phoneme is the basic unit of human speech. It very roughly corresponds to a letter in the English alphabet, although there are cases where one letter is actually multiple phonemes (a long 'a') and a phoneme is represented by multiple letters ('ts', 'ch').
You can have voiced phonemes ('v'; vocal chords vibrate) and unvoiced phonemes ('f'; vocal chords do not vibrate, unless you're saying it wrong, like "fuh" or something).
The voiced phonemes have attributes associated with them: voice onset, voice segment and voice offset, among many others. The voice onset is the point in time where the vocal chords begin to vibrate (I have a little piece of useless trivia about Voice Onset Time, see below). The voice segment is the middle part, where you're holding the sound. It can be long, short, medium, just right, etc. And the voice offset is the trailing end.
We know that Chinese is a tonal language. We also know that there is a "second tone" which is a rising tone. To make this tone quality, you would make your voice rise, much like you would when you're asking a yes-no question in English.
How, then, do the Chinese differentiate between a second tone that is asking a question versus a second tone that is making a statement? That would be the 64 Million Dollar Question of the day... well, of this post, anyway =)
For example, the phrase 明白 (míng bái), ends with a second tone. As a question, it's "明白﹖" As an answer (which, in this case, is a statement), it's "明白。"
It's very very subtle, but there is an intonation portion at the voice offset of "bái".
In the question, the quality of the second tone is tending toward a continued rise as if your voice were going to follow through with the implied interrogative particle "嗎" (which is a high neutral tone that sounds very similar to the first tone). Example: 明白嗎﹖ (míng bái mā?).
In the answer, the second tone quality is as if your voice were going to follow through with the implied perfect/completive/modal particle "了" (which is a mid, steady neutral tone if it follows a second tone). Example: 明白了。 (míng bái le).
If you wanted to get technical about it, this alternate second tone is a separate tone. It is distinct and it has meaning assigned to it.
When people say that Mandarin is easier because it only has four tones and Cantonese has seven, that's not true. Cantonese only has seven because they've decided to differentiate seven of them. If we differentiated all the different tones in Mandarin, it's on par.
Mandarin has many many more tones than just the four. There are (linguistically speaking) at least 8: the four that you're taught, I count at least three different flavours of the neutral tone and this alternate second tone. I'm positive there are more of these subtle ones that come up in different contexts and circumstances that are lumped into one of the regular four. In fact, I know there are ones that waver in intonation, we purposely fluctuate the tone a little.
Native speakers will tell you that's not true and that there are only four tones. Some will deny that even one neutral tone exists. Many certified Mandarin teachers don't understand stuff like this. My mother has been teaching adult and children's Mandarin and Cantonese classes for over 40 years and she is still finding out in dribs and drabs that I was right about a lot of these things.
Little Piece of Useless Trivia
The consonant 'b' sound does not actually exist in English. Neither does the 'd', for the same reason.
Because the voice onset time is so late, what we're really saying is an unaspirated 'p' for 'b' and an unaspirated 't' for 'd'. The English 'p' and 't' are aspirated 'p' and 't' which means we put an extra puff of air into it. An English 'b' is a 'p' without the extra puff of air, likewise for the 'd'.
In many European languages (and you Europeans on my list, correct me if I'm wrong), a 'b' is actually a 'b'; the voice onset is much earlier than the English 'b'. At least, I think it's different in French. "quatre bières" versus "four beers" sounds different to me.
My highschool Physics teacher, Evan J., spoke Afrikaans which is very similar to Dutch. He would always say 'b' and 'd' on words that begin with 'p' and 't'. It was fun talking about "backets of energy", "photon dorbedos" and "Desla goils". =)