half life is how long it takes for half of a (radioactive) substance to degrade. the thing about radioactivity, is that, it doesn't decrease by a linear rate, it works more with the principal of a reverse exponential rate. i dont know the technical terms for that but it means, if you have 100 particles (with an 8 day half life) then at day 8, you have 50 particles, but at day 16, you dont have 0 particles, you have 25. at day 24, about 12. at day 32, about 6. day 40, 3 particles, and so on into the wild blue yonder. hope that helps