Why does Trump want a wall on the US-Mexican border?
Trump says a "big, beautiful wall" will deter illegal immigrants who have taken away white-collar jobs from the American people.
Around 11 million illegal immigrants live in the US. Nearly half of them or 6 million are Mexicans.
However, more Mexicans are leaving the US than are coming to live there.
Illegal Mexican immigrants numbered 5.8 million in 2014, down from 6.4 million in 2009.
But isn’t there a fence already on the US-Mexico border?
The border that stretches from the Gulf of Mexico to the Pacific Ocean is 1,954 miles (3,145 kilometres) long.
It traverses deserts, rugged mountains and the Rio Grande river that work as natural barriers to the movement of migrants.
A stretch of 650 miles is already covered with metal fences, barbed wire, steel barriers and concrete slabs.
Some areas are intensely fortified with triple barriers such as the section that separates the urban areas of San Diego in the US state of California from Tijuana, in the Mexican state of Baja California.
In some places, it is blocked by thick metal plates 3 metres high.
The US border patrol also uses cameras and drones to monitor the movement of migrants.
The existing fence cost Washington $7 billion.
What's different about Trump's wall?
Trump wants a concrete wall to be built that would be roughly 1,000 miles long.
"It's not a fence. It's a wall," Trump said at his first press conference.
He also wants it to be 30-feet high.
What are the challenges in building this wall?
The border runs through a remote desert in Arizona, over rugged mountains in New Mexico, and for two thirds of its length along rivers.
Building a wall along some sections of the border would be difficult and expensive.
A 40-foot tall concrete wall – a probable scenario drawn by experts – that also runs 7-feet below the ground, with a thickness of 10 inches, would require millions of tonnes of building equipment.
Around 17 million tonnes of concrete and 2.4 million tonnes of cement would go into building the structure, according to a structural engineer whose calculations have been widely cited.
The construction material needs to be brought from factories over long distances. Thousands of workers would be needed to build the wall.
How much would it cost and who pays for it?
Trump says his wall would cost between $10 billion and $12 billion.
But The Washington Post says that number is underestimated and the actual cost could go as high as $25 billion.
Trump believes he can make the Mexicans foot the bill for the wall through various economic actions.
And he has suggested outrageous ways to achieve that goal, such as stopping the flow of money Mexican workers send back home, to increasing visa processing fee and raising tariff on Mexican goods.
Mexico has rejected the idea of a wall on numerous occasions.
Mexico’s President Enrique Pena Nieto said his country won’t pay for it.
"Mexico doesn’t believe in walls,” he said in a televised address.
Can Trump force Mexico to pay?
If Trump went ahead with his threat of imposing tariffs on Mexican goods, he will violate America’s own law and multiple international treaties, including the United Nations' charter.
Both the US and Mexico are part of the North American Free Trade Agreement (NAFTA), the important trade alliance that also includes Canada.
Any unilateral imposition of tariffs on imports of Mexican goods could be violation of the agreement.
The move could also be challenged under the Vienna Convention on the Law of Treaties. But as president, Trump has the power to bypass Congress and impose tariffs.
Impounding the money illegal immigrants send back home won’t be easy.
Banks could be forced to curb the transfers, just like the flow of money to finance terrorism is restricted. This will obviously discourage banks from handling the remittances.
Money transfer companies like Western Union could be made to ask immigrants to present their legal documents.
In that case, migrants could resort to sending money back home through illegal money transfers, where an agent in one country takes cash and their partners pay off the family in the other against a commission without an actual transfer taking place.
The proposal to raise fee on visas and border passing cards could attract similar measures from Mexico, which is the top tourist destination for the US citizens with 26 million visitors per year.
Would it hurt the economies of the two countries?
Both countries would take a significant economic hit.
Mexico is US’s second largest export market and the third largest trading partner.
US exports to Mexico surged drastically to reach $240 billion in 2014 from $41.6 billion in 1993 because of NAFTA.
Similarly, US imports from Mexico jumped to $295 billion from just $40 billion during the same period.
American made car parts and electronic circuits go to Mexico where they are assembled into finished goods and imported back.
The wall will also hurt one of Trump’s own constituencies – farmers.
More than half the number of men and women who till American soil are undocumented farmhands. When the flow of desperate Mexican job- seekers stops, so would the profits for the farmers.
Will the wall endanger the lives of migrants?
Research shows hundreds of people die every year trying to cross the border since US tightened security in the 1990s.
Stricter control pushed migrants to take a route through the Sonora Desert in Arizona, leading to more deaths.
Authorities in Tucson, Arizona, recorded the highest number of migrants who died trying to cross over to the US.
It registered the deaths of 2,238 migrants between 1990 and 2012. The pro-migrant Defense Coalition puts the death toll at more than 5,000.
Since 1999, on average 163 migrants have died every year. That is significantly higher than the average of 12 deaths recorded between 1990 and 1999, before tougher security was put in place.
Most people die due of exposure to heat as they try to cross the remote areas of Arizona.
At some places the wall is marked by hundreds of crosses for each of the hundreds of men, women, and children who have died while passing through the burning desert.
Can migrants get through the wall?
Over 90 percent of the illegal migrants get help from human traffickers.
Tunnels are usually dug by drug smugglers but if the wall raises the cost of human trafficking, this route could also be used.
High tech sensors that detect human movement across a fence would be hard to maintain on such a large scale. False alarms triggered by animals is another problem.
Another possible option could be concealment in freight trucks.
More than 5 million trucks cross between the two countries every year – nearly 50 percent of them in the two US states of Texas and California that together host 53 percent of illegal immigrants.
Who stands to benefit from the wall?
The companies that supply raw material for the wall.
Cemex, the Mexico-based building material manufacturer, saw its share price at the Mexico’s IPC stock exchange surge to an eight-year high on January 20 as Trump made his inauguration speech.
The company’s share price, a barometer of future earnings, has already jumped 82 percent in 2016 on hopes that its cement could go on to building the wall.
The Mexican company, which has factories close to the border, even offered its services to Trump.
More than $700 million worth of concrete and $240 million of cement could be used in construction of the wall.
But the interest in Mexico-based Cemex might fade. Trump has indicated that he preferred American companies getting the contract for infrastructure projects.
And so a rally has started in share prices of American companies after the president issued the executive order for construction of the wall.
The majority shareholders of companies such as Martin Marietta Materials and Vulcan Materials must have been pleased to see such a rise in their worth.