Walls work to keep people out. And so Trump's border wall may stop some human crossings.
But lots of Americans are begging for a border wall to keep opioids out. Because the world has this terrible opioid crisis, which has already killed 8,000 people since you started reading this sentence.
Problem, experts say, is: drugs are small.
Drugs don't have to jump walls. Or trek through the desert. They just have to snuggle into a nook of the thousands of cars that cross the border every day.
"Most of the drugs come through regular checkpoints," said Sanho Tree, an international drug policy expert.
Through these checkpoints, clever drug smugglers have never had trouble getting a kilo of heroin.
Plus, drugs are getting smaller.
Fentanyl, which feels a lot like heroin, is infinity times stronger. Smugglers can hide a year's supply of fentanyl inside a fake silica gel packet shipped with a cell phone case. Carfentanil, is infinity-plus-one times stronger than that. Put a kilo of carfentanil in your car's reservoir of windshield washer fluid, drive it through the border and you can get half of El Paso high for a month.
"Fentanyl and its analogues are so easy to smuggle, they're so compact," said Tree, "Even if the wall does stop heroin, which it won't, they'll adulterate with fentanyl of carfentanil. They're so compact it's impossible to stop. Then you're dealing with many times more potent and problematic drugs."
So if Trump succeeds in building the $8 to $27 billion wall stretching 2,000 miles — which he probably won't — the opioids will still come, they'll just be the less safe ones. Instead of organic heroin from Mexico, you'll get synthetics.
Building a wall to keep out opioids is like putting up a cargo net to stop the flow of fruit flies.
"I don't care if we build a wall we are not going to stop the flow of those drugs," said San Miguel County, Colorado's Sheriff Bill Masters, Colorado's longest-serving sheriff and author of "Drug War Addiction: Notes from the Front Lines of America's #1 Policy Disaster."
"We cannot even stop the flow of fentanyl coming from China, and it's got to come across a 4,000 mile stretch of ocean."
Can Trump build a wall across the ocean, too?