When to use static methods?

I am aware that we use static method when its behaviour is same irrespective of how many objects gets instantiated from same class.Suppose if i have a class Test with method add

Class Test
{
public int add(int a, int b)
{
return a+b;
}

}

For the above example is it good to use static keyword or not? Can someone explain me with good reasoning.

Quote:

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behaviour is same irrespective of how many objects gets instantiated

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Your reason sounds good.

Are there any other reasons as when to use static method vs when not to use. does static method takes more memory?