The half-life of a nuclide is the length of time required for half of the nuclides in a sample to decay.

Imagine a table full of coins. On one side they are yellow and on the other side gray. At an initial time t0, they all show their yellow side. After one half-life (t1/2) we flip them all. Half of them will remain yellow side up, the other half gray. After another half-life we flip the yellow ones again, and half of those will also turn gray, leaving only 25% yellow remaining.

The image shows four stages. The first is a set of 16 coins all yellow side up. This image is marked t 0. An arrow connects this stage to the next one which shows that half of the coins have been flipped leaving 8 remaining. This stage is marked t half. An arrow connects this stage to the next one which shows that half of the remaining coins have been flipped leaving 4 left yellow side up. This stage is marked 2 t half. Finally, the last stage shows that half of the remaining coins have been flipped once more leaving only 2 yellow ones remaining. This stage is marked 3 t half.

Let's imagine the yellow coins represent our nuclides, after each half-life, half of the remaining nuclides undergo radioactive decay events. After another half-life, half of the remaining nuclides also undergo decay events, leaving only 25%. This process continues, halving the remaining active nuclides each time until there are no remaining active nuclides left, and stable daughter nuclides have been reached.