The nucleation reaction can start with any heterogeneous surface, such as rock salt, but Mentos have been found to work better than most. Tonya Coffey, a physicist at Appalachian State University, found that the aspartame in diet drinks lowers the surface tension in the water and causes a bigger reaction, but that caffeine does not accelerate the process. The geyser reaction will still work even using sugared drinks, but diet is commonly used both for the sake of a larger geyser as well as to avoid having to clean up a sugary soda mess.