Today I was having lunch and I ordered plain soda. And some amount of sugar was added, suddenly all the liquid fell of the glass. I thought there should have been a reaction between plain soda and sugar. What is it?
Soda is carbonated water - it has carbon dioxide dissolved under high pressure. Carbon dioxide is a gas that would like to escape. Shaking the bottle of soda, for example, provides a suitable mechanism to allow release of the gas.
Adding dry sugar also provides a mechanism for this to occur through nucleation. The sugar crystals give the carbon dioxide a surface via which to escape the solution. This is not a chemical reaction per se, but a physical reaction. It is essentially the same process as the coke-mentos reaction. Sugar crystals are great for this, as they have a rough surface and therefor a very high surface area available.