Whatever you do, don't heat the mercury. The main way of getting poisoned by mercury is by inhaling the vapours. Your brother is more right than your teacher. Mercury is very poisonous but it has to get inside you. Touching it a little bit will not allow much at all to get inside, but do not handle it a lot. Wear gloves. Make sure you are careful to wash your hands afterwards. Do not play with mercury while eating. Mercury affects the nervous system and will make you go insane if enough of it builds up inside you. It accumulates inside your body over time so the less exposure you have the better. I think a little fun with mercury is OK, but be careful. The Mad Hatter in Alice In Wonderland went nuts because hat-makers used mercury to make hats in the olden days. You can find out a lot about mercury and its reactions at WebElements.