Stephen Voltz and Fritz Grobe have turned chemistry into a viral video sensation. Their experiments mixing Mentos candies with Diet Coke have produced a series of online videos that have logged more than 40 million downloads and shaken up marketing at the Coca-Cola company. This week we snagged the duo, whose venture is known as Eepybird, and asked them about the secrets of their success. A hint: Don’t overdo it on production values, but pay attention to staging. Above all, keep it genuine.