There is a myth that drinking Diet Coke while chewing Mentos causes your stomach to explode, though this has been disproved by MythBusters and Snopes. Both sources say that the reason for the explosion between Diet Coke and Mentos is because of the outside of the Mentos reacting with the Diet Coke. If the Mentos are still in your mouth, the Diet Coke will most likely explode, but if they are in your stomach, they won't cause an explosion, but they still may cause a lot of burping. Still, none of those sources are very reliable, so I'm going to have to prove how it works chemically.
This study testing the reaction states that
"It is clear...that the roughness
of the sample is a major contributor to the explosiveness of
This Steve Spangler Science experiment page also agrees that the surface of the candy is an incredibly important factor in the Diet Coke/Mentos reaction.
When you drop the Mentos into the soda, the gelatin and gum arabic from the dissolving candy break the surface tension.
It is clear that is indeed the surface of the Mentos that causes such a reaction. So, it can be seen that if you eat the Mentos and they are in your stomach, the surface of the Mentos begins to break down, so it makes sense why there would be just burping, rather than an explosion (like in the YouTube video above). If the Mentos are still in your mouth, the surface has probably been already slightly broken down, but there would still be an explosion, just to a slightly smaller extent.