Question

why is it easy to hold a Steel
tumbler with cold milk than the one with hot milk

Answer 1

The tumbler is made up of steel which is a good conductor of heat.
As the milk is hot, the tumbler transfers the heat of the milk into our hands thus we feel pain if the milk is too hot. 
Thus, if the milk is cold, than the tumbler is easy to hold.

Answer 2

Answer:

The tumbler is made up of steel which is a good conductor of heat. As the milk is hot,  the tumbler transfers the heat of the milk into our hands thus we feel pain if the milk is  too hot. Thus, if the milk is cold, then the tumbler is easy to hold.