Probabilities and Certainties

When I was in psychology 101 at Bradley University, Dr. Smith told us about his experiment with predicting heads or tails in coin tosses. He said that he and some others took 1,000 pennies and flipped them one by one and then tallied the results. And the result was that there were 500 heads and 500 tails. I don't really remember what he was trying to teach with that anecdote, but it stuck in my memory as the gospel truth.
Fast forward to today and the problem of certainties. For example, the saying goes that the bread always falls jelly-side down. I've had occasion to test that out, and I'd say it was more like an 80% likelihood, but others would argue with me. I'm not about to get 1,000 slices of jelly bread to try it out.
I do have one certainty, however. When you put your cup of coffee in the microwave and start it up, the turn table will revolve and will come to a stop with the cup on the far side of the oven. Jim and I have tested it hundreds of times, and it always works out that way!