You've committed a crime. The cops are after you. You're on the run. How can you disappear, vanish into thin air?

That's the question Philadelphia police must be asking themselves as they hunt Derrick Hershal Rollins, 24, chief suspect in the random shooting of John Le, 29, on July 29 in Haverford Township.

We are told that criminals are dumb, and there are countless examples to prove that, but they often know things the honest Jamal or Jane would not know.

Like disappearing, for instance, vanishing from the grid, as Rollins has done, even while police raid places he is known to have been.

Police believe he has remained in the area because "he has strong family ties, he has a young child with a young lady, and is dating another young lady," I'm told Monday by Lt. John Walker of Southwest Detectives. "We believe he has no money."

Walker suspects that Rollins is getting help from friends.

In the trade, that's known as "couch surfing," says Marc Bourne, co-founder of the Know It All Intelligence Group, a private-eye firm in Bensalem. The suspect may bunk in a friend's basement or on a couch.

"He is a fugitive," Walker warns, "and if they get caught helping him, obstruction of justice has a very serious penalty."

Bourne says, "In police work the big thing is finding out who the associates are. Word on the street is like Facebook: Somebody is going to know something."

That gives Rollins reason to run, and it doesn't take much money to jump on a bus or train, and if he pays with cash, there would be no trace of it.

Speaking of trace, isn't that something the FBI does routinely?

Yes, it is, but FBI spokeswoman Carrie Adamowski declined to comment on disappearing techniques, saying, "We don't want to give criminals looking to go off the grid any ideas on how to evade capture."

Understandable, but the real baddies already know how to do it and the less-experienced crooks can find guidance on the web.

Just as I did.

What follows isn't a primer for aspiring felons, it's a roundup of techniques that are pretty much common knowledge:

  • If you have a car, you won't want to drive it (unless you switch the license plate). Stay away from taxis and Uber, because they record where you were dropped off.
  • You may love your cellphone, but it's a beacon to your location. Get a prepaid phone that you will use and toss, just like the terrorists. Don't call friends and family, even with a prepay, because your call can be traced to a location.
  • Same thing with credit cards, ATM machines, and even your checks. It's cash and carry for fugitives. You should have prepared a cache of cash before your decision to vanish. Preparation is essential and can take months or longer, says Bourne.
  • Avoid public places with a lot of closed-circuit security cameras. Such camera-free oases are getting rare.
  • Change your appearance in a dramatic way. If your hair is long, cut it short. If it's short, use a long wig. Change the color of your hair. Bourne says hats, sunglasses, and hoodies are all effective in covering your face.

In the end, though, Bourne expects Rollins to be captured soon.

So do I, because most fugitives are.