Depending on how hot your neck of the woods is, gallium is the only metal other than mercury to be in a liquid state at room temperature. Luckily, it is also completely safe to handle. The combination of these two properties has made gallium a popular gift for the geek in all of us. Unfortunately, it isn't the cheapest and, unlike mercury, it will coat most surfaces which is quite messy. Also, storing it in glass is inadvisable as it expands when it cools down into a solid and will easily shatter it.