I do not quit understand the concept of half life. If I have something with a half life time t = 1 day than after 1 day only half of the stuff is left? So if I have 1 kg I would have one day later 0,5 kg but if I started out with 10 kg I would end up with 5 kg? How does the substance "know" how much of it is around?

Question

anonymous · Answer

Half life is based on the concept of radioactive decay. Unstable isotopes have a probability of decaying (ie changing into a different form) depending on the isotope. Half life measures how much time must elapse such that the probability that the isotope has decayed is 1/2. This is a statistical probability so when you have a lot of isotopes (enough to see macroscopically) this half life is accurate and very useful.

anonymous · Answer

The answer above is good, but to question "How does the substance "know" how much of it is around?" it maybe helps if you look at the animated image on the right: http://en.wikipedia.org/wiki/Half-life#Probabilistic_nature_of_half-life and read the chapter in Wikipedia.

anonymous · Answer

So there is not just one half-life of an substance X because the half-life is a function of the mass of the substance?