There are three flipped cups, only one has a red ball inside, others are empty. After your selection one empty cup will be lifted and you can stay with your choice or switch cups. Make a couple dozen* experiments switching you choise, take a look the ratio value. Then reset the counter and make experiments again without changing the choice. If you are not familiar with Monty Hall problem, you will be surprised the results difference.

*More experimentation may be required to observe the estimated difference in results because the effect is based on probability theory.