https://astralcodexten.substack.com/p/the-unbearable-semiheaviness-of-being
I hear that Google tests prospective employees with weird vaguely-science-related riddles. If I were in charge of this, here's what I would ask:
You're an American spy in Cuba. The CIA has gotten you a position refilling the water coolers in Castro's presidential palace, hoping you can poison him. But Castro's security is pretty good. Every time you enter the palace, they search you so exhaustively that you're sure you can't smuggle anything in. And you're sure you can't access any poisons within the palace. And every time he drinks water, Castro calls in a chemist to test it for any impurity first; the tests can detect any contaminant at any concentration. On the plus side, you're completely unsupervised within the palace, and have access to a kitchen with all the usual kitchen appliances. And the CIA has given you a time manipulation gizmo, so you can take literally as long as you want, even if it’s thousands of years. How do you kill Castro?
One answer: Start with an amount of water several thousand times Castro's usual daily consumption. Put it in the freezer until it's half frozen. Dump out the unfrozen half, melt the frozen half, then repeat this process with the meltwater. After some very large number of cycles, put the result in Castro's water cooler every day. He'll be dead within a year.
The hydrogen in water is a combination of normal hydrogen (only a proton in the nucleus) and deuterium (a proton and neutron in the nucleus). These have slightly different chemical properties, so you can do various types of distillation to enrich for one or the other, including repeated freezing (realistically freezing works very slowly; our hypothetical spy would need an unrealistic amount of time, water, and patience). Normal water is about 99.9% H2O, 0.1% HDO, and negligible amounts of D2O. Water with more D2O than normal is called heavy water, water with more HDO than normal is called semiheavy water, and water with more H2O than normal (ie not even the usual tiny amounts of the other two constituents) is called light water.
