Happy Monday to you.

I'm Carl Azuz.

And this is CNN STUDENT NEWS:

In a few minutes, we're going to explore a court case involving President Obama's health care reform law.

But we begin today in South Korea. President Obama and officials from more than 50 countries are there right now for a meeting about nuclear security. They're going to talk about things like nuclear terrorism and how to keep nuclear material from around the world safe.

But before the meeting started, there was some tension regarding North Korea. That nation says it's planning to launch a satellite on board a rocket next month. That kind of launch would violate a deal that North Korea made with the U.S. recently, as well as a United Nations Security Council resolution.

President Obama warned North Korea that a missile launch will not be rewarded. On Sunday, the president also made a visit to Korea's demilitarized zone, the DMZ. This is the region that splits North and South Korea. The president was there to meet with some of the 28,000 U.S. troops that are stationed in Korea. He also kept up a tradition carried out by previous U.S. presidents.

Paula Hancocks has more on that.

Former U.S. President, Bill Clinton, described it as "the scariest place on earth" when he visited in 1993.

Ironically called the demilitarized zone, the border between North and South Korea is actually the most heavily fortified border in the world. Two-and-a-half miles or four kilometers wide, much of the DMZ is a no man's land buffered by watch towers and land mines.

The Joint Security Area is where the 1953 armistice was signed between the North and the South. It's where negotiations take place today. And it's the one place U.S. Soldiers can see their North Korean counterparts up close.

It's a tourist site for visitors to South Korea, and, of course, for American presidents. Ronald Reagan, the last American leader of the cold war, visited the final frontier of the cold war in 1983.

His assessment...
It looks like a Hollywood back lot.

Previous U.S. President, George W. Bush, visited the DMZ in February 2002, just weeks after he branded North Korea as part of his so-called "axis of evil," along with Iran and Iraq.

So why do presidents feel the need to visit one of the most tense borders in the world?

There is a signal that's sent to North Korea regarding U.S. Resolve and the strength of the alliance. There's a signal sent to South Koreans that the U.S. Is reassuring them and that it will fulfill their alliance commitments.

And just in case you thought the VIPs were just on the South Korean side, the new North Korean leader, Kim Jong Un,visited the DMZ earlier this month, standing just meters from South Korean territory and ordering his troops to be on their highest alert.

Next up today, the race for the Republican nomination. There are no primaries or caucuses scheduled for this week.

The next ones aren't until April 3, but the candidates did face-off over the weekend in Louisiana.

Former Senator Rick Santorum was the winner there. He got around half the votes in the primary and picked up at least half of the delegates that were at stake, too.

For the candidates, that's what these primaries and caucuses are all about -- winning delegates. This year, it's going to take 1,144 delegates to win the Republican Party's nomination. Former Massachusetts governor, Mitt Romney has the most delegates and more than twice as many as Senator Santorum.

We reported on the death of Trayvon Martin in Florida last week. The 17 -year-old was shot and killed by George Zimmerman, a neighborhood watch captain. That happened in February and authorities say Trayvon Martin was unarmed.

George Zimmerman told police he acted in self-defense and you can go to the video archives on our home page and look at last Thursday's transcript for more details on this story.

It has led to protests and rallies across the country, like this one you see from Virginia. One person who was at a rally in the state told CNN that part of the reason he thinks there are so many protests is because people think this shooting was senseless.

In other states, like you see here in New Jersey, people took part in marches. You might notice a lot of them wearing hoodies. That's what Martin was wearing on the night when he was shot.