Qing Wen!

In the process of encouraging more Americans to study Spanish rather than Mandarin, Nicholas Kristoff notes that in Mandarin

there are thousands of characters to memorize as well as the landmines of any tonal language. 

How true! How true! Kristoff shortly proves the latter point in more ways than one:

The standard way to ask somebody a question in Chinese is “qing wen,” with the “wen” in a falling tone. That means roughly: May I ask something? But ask the same “qing wen” with the “wen” first falling and then rising, and it means roughly: May I have a kiss?

Just one possible reaction if you use the wrong tone.

Kristoff is right, so long as you don't mind sounding like a speech synthesizer. The classic description of third tone is a falling tone followed by a rising tone, but in practice it is relatively rare to pronounce the second half (the rising tone), particularly in fluent speech (in Taiwan, anyway; China has a lot of regional variation in Mandarin, so I don't know whether this holds everywhere). Figuring out when to pronounce the full tone and when not to is just one of many issues L2 Mandarin speakers run into.

Actually, third tone is worse than I just suggested. Qing wen is actually a good example, because the qing is also in third tone. When there are two third tones in a row -- as there are in the qing3 wen3 that  means "may I ask you a question?" (I'm writing in the tones with numbers here) -- the first one is pronounced as if it were second tone (start low and rise high). So even though qing technically doesn't change, its pronunciation depends on which wen you are using.

If you have three or more third tones in a row (e.g., ni3 you3 hao3 gou3 gou3 ma0?), deciding which syllables will be pronounced as if they were second tone is a complicated issue. I'd explain it to you, but I don't actually know myself. I've been told you actually have some flexibility in what you do, but I'm not sure that wasn't just another way of saying, "Sorry, I can't really explain it to you."