"IAEA scientists have described polonium 210 as 250bn times more toxic than hydrogen cyanide. A cube of the isotope, measuring 0.35mm square and weighing 400 micrograms, which would sit on the full stop at the end of this sentence, would contain 3,400 lethal doses. The internal report also describes the medical effects of the isotope. Its half-life - the time required for half the mass of a radioactive substance to decay - is 138 days. But the effective half life in the body is 30 to 50 days, because the body expels a proportion of the material. It attacks first the spleen, then kidneys and liver, bone marrow, lymph nodes and the respiratory tract."
A stone is not inherently dangerous, but if one bangs in on one's head for a day or so in may be reclassified. A wall is also pretty safe unless one bangs one's head against in for a while.
Nothing is foolproof when fools are so clever.