The sum of the probabilities of all of the possible events should be equal to 1.

Take the coin and flip it twice. What is probability to get twice head?

There are 4 situation, 1H and 1H, 1H and 1T, 1T and 1H, 1T and 1T.

Then there are 1/4 chance to get the conin and flip it twice get both head.

Every flip is an independent event.

The both independent probability can * each other make what is probabilities to get both situation related.

Probabilities to get the T first, H second, T third is each independent probabilities can *. So it is 1/2*1/2*1/2=1/8.

Every probabilities is an INDEPENDENT event.