Washington ? Most kids have seen a presidential election or two and they know why it’s such a big deal – the person voters choose every four years to lead the United States has a lot of power.

But another part of the government, the U.S. Supreme Court, is just as powerful – and replacing one of the court’s nine justices is, in some ways, a bigger deal. The most recent vacancy was filled in 1994.

A justice can keep the job for life, and spots open up only when a justice dies or steps down. That’s happening now: Justice Sandra Day O’Connor is retiring after 24 years on the court. On July 19, President Bush nominated Judge John G. Roberts Jr. to replace her.

Questions and answers about the nation’s highest court:

Q: What do Supreme Court justices do?

A: Their main job is to interpret the U.S. Constitution, the blueprint for our system of government, written by the country’s founders. The Constitution establishes people’s basic rights, including the right to free speech, a fair trial and the religion of their choice. The Supreme Court has the final say on what’s “constitutional.” That’s a tricky task partly because most of the Constitution was written more than 200 years ago, so the current justices sometimes have to decide things that the country’s founders never even thought of. For example, are Internet searches a form of free speech?

There’s no doubt that the Supreme Court decides some of the most important, and controversial, issues facing our country. The court’s decisions can change history – and the way we all live. Before a Supreme Court ruling in 1954 called Brown v. the Board of Education, it was legal to have only white kids in one classroom and only black kids in another. The court said that system of segregation violated the Constitution.

Both the executive and legislative branches of government decide who will be appointed to the U.S. Supreme Court. Justice Sandra Day O'Connor has announced her plans to retire.

Q: Who decides who becomes a Supreme Court justice?

A: This job, in the judicial branch of our government, is so important that the two other branches – executive (the president) and legislative (the Congress) – have a say. The president nominates someone, who then must be approved by the U.S. Senate. If Roberts is approved, he could be on the court when its new term starts Oct. 3.

Q: Why is there so much interest in who gets O’Connor’s seat?

A: The court decides many cases on 5-4 votes. O’Connor often cast the deciding vote and was a pretty fair tie-breaker. That role could fall to the new justice.

Q: What kind of justice would Roberts be?

A: No one knows for sure. He is intelligent and has argued cases before the Supreme Court 39 times. People are reviewing everything he has ever written or said to try to figure him out. But trying to predict how a justice will rule is often impossible. History has shown that justices don’t always vote the way the presidents who nominated them thought they would.

Also, justices tend to serve for many years. Over time, people’s views can change. Ask your mom and dad if they have the same opinions on issues today that they had in 1981, the year O’Connor joined the court.