Why a glowing bulb breaks when a drop of water fall on it

The entire bulb becomes hot when it glows and when a drop of water falls on a small area of the hot bulb, only that area cools down and contracts while remaining area is still hot.  This forced contraction in small area makes the bulb break This is due to thermal stress.

this happens due to thermal shock. when you put drop of water on some part of glowing blub that part cools down and shrink,while other parts are hot. this will cause very large thermal stresses, which finally breaks the bulb.

Due to differential heating of inner and outer glass surface of bulb

The glass of a glowing bulb is hot. When water falls outside, it cools down outside, but as glass is a bad conductor, it remains hot inside. Due to uneven heating it undergoes stress and breaks.