Sit back, relax, and read about Linda:
Linda is thirty-one years old, single, outspoken, and very bright. She majored in philosophy. As a student, she was deeply concerned with issues of discrimination and social justice, and also participated in antinuclear demonstrations.
Now, which alternative is more probable?
1. Linda is a bank teller, or
2. Linda is a bank teller and is active in the feminist movement.
If you selected the latter, you've just blatantly defied logic. But it's okay, about 85 to 90 percent of people make the same mistake. The mental sin you've committed is known as a conjunction fallacy. Think about it: it can't possibly be more likely for Linda to be a bank teller and a feminist compared to just a bank teller. If you answered that she was a bank teller, she could still be a feminist, or a whole heap of other possibilities.
A great way to realize the error in thought is to simply look at a Venn diagram. Label one circle as "bank teller" and the other as "feminist." Notice that the area where the circles overlap is always going to be smaller!