Magnesium/Silver Nitrate Explosion

A single drop of water is added to a mixture of powdered magnesium and silver nitrate. This initiates an explosive displacement reaction, producing magnesium nitrate and silver. Reactions like this were used in early photo flashes.
This is sometimes called a "competition reaction" and occurs because the more reactive element (in this case magnesium) displaces the less reactive one (silver) from a compound (silver nitrate). Here is the word equation for this reaction:
magnesium + silver nitrate → magnesium nitrate + silver
The drop of water does not form part of the chemical equation. The water speeds up the reaction by dissolving the silver nitrate, and therefore dramatically increasing its surface area. Reactions between two solids are very rare. This is because the solid particles touch in very few places. Imagine two touching snooker balls: only a tiny percentage of these surfaces are actually in contact.