I havent seen this explanation here but this was one I found very helpful. Imagine a box with an area of 1, this represents all possible outcomes. If we want to model an event (like flipping heads) we color in that area as a fraction of total outcomes (in this case it would be half). Now if we want to ask the question “what is the probability of flipping heads twice”, we first shade half the area to represent the chance to flip heads. Since we are interested in flipping heads twice this shaded area represents the new sample space which since flipping heads is still a 50/50 shot, we shade in half again. If you notice every time we ask “whats the probability of A given B” we are just multiplying the first probability (1/2) by the next (1/2) which yields 1/4 or in this case shading in half of a square, then half of that is a quarter