Gallium- the gift you can buy for “someone who has everything”. This is so cool and I have definitely not met anyone who has it! If you hold a solid gallium crystal in your hand, your body heat will cause it to slowly melt into a silvery metallic puddle. Pour it into a dish, and it freezes back into a solid. It’s non-toxic too! It’s a metal which in it’s elemental form is not found in nature. Used as a semi-conductor, gallium is a unique ‘liquid’ to play with and would certainly entertain your scientific-minded friends! You can actually buy it on Amazon!