This does a good job of explaining the answer to your question. Basically, everyone has two sets of genes from their parents. When we reproduce, we pass half of them. But…we don’t know which half. So you could pass your mom’s gene that causes blue eyes with one child, and your dad’s gene for brown eyes with the second child. This is what they mean when they say that you have a 50% chance – just think of the decision of which gene you get being based on a coin flip. Each time it flips…there’s a 50% chance that it will land on mom or dad. Considering the number of time the coin gets flipped, the average works out to about half of one, half of the other.