Question

why does gallium starts melting on our palm?

Answer 1

gallium starts melting in hand because the melting point of gallium is lower than the temprature of hand hence it starts to melt in hand

Answer 2

Like most other metals, Gallium is solid at room temperature (or liquid if it is too hot inyour room). But, if it is held [in hands] for long enough, it melts in your hands, and doesn't poison you like Mercury would. This is because of its unusually low meltingpoint of (~29 degree Centigrade).
hope it might help you