This hypothesis gained further support when rock salt was used as a "jump start" to the reaction. A paper by Tonya Coffey, a physicist at Appalachian State University in Boone, North Carolina, confirmed that the rough surface of the Mentos candy helps speed the reaction. Coffey also found that the aspartame in diet soda lowers the surface tension and causes a bigger reaction, but that caffeine does not accelerate the reaction. On the other hand, it has been shown that a wide variety of beverage additives such as sugars, citric acid, and natural flavors also enhance fountain heights. In some cases, dissolved solids that "increase" the surface tension of water (such as sugars) also increase fountain heights. These results suggest that additives serve to enhance geyser heights not by decreasing surface tension, but rather by decreasing bubble coalescence. Decreased bubble coalescence leads to smaller bubble sizes and greater foaming ability in water.