The Mentos do not explode. CO2 comes out of solution on tiny imperfections in a surface. Look in your glass of soda - see the stream of bubbles coming from a single point? The surface of the candy is covered in thousands of similar imperfections, so dropping it in the cola makes thousands of streams of bubbles. These bubbles expand as they form, and as they are expanding at the bottom of the cola, the cola above the bubbles is pushed violently out of the bottle. (That's why you need to use diet cola - sugary stuff gets everywhere sticky.)
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Look at; www.ehow.com/how-does_5131947_coke-mentos-explode.html
According the Mythbusters it is nucleation. When the Mentos is added to a carbonated drink, the surface of the mint is littered with many small holes, allowing CO2 bubbles to form very rapidly and in great quantity resulting in a jet of foam.
Though the same thing happens if you add other sugars to other carbonated drinks -- the sugar dissolves and drives out the CO2. The advantage of mentos appears to me to just be that they dissolve at the right rate to sustain the reaction for a longer time.
+1. Love mythbusters!