Sanjay how many times your teacher said you but also you didn't understand. it is actually a property of water to expand when it goes below the temperature of 4degrees to 0degrees. each and every thing has its characteristics for example abook has its characteristics of having pages or a plant has a property of absorbing water or sunlight. therefore it is a property of water to expand when it goes below the given temperature. dont wirry about it . it's all god made. but also it is an interesting subject. Bye friend. I know you are a scientist.