little interaction

“Hmmm?” Sir Stuart said. He glanced down and saw what I was talking about. “Oh. I suppose, yes. I stopped noticing it after seventy or eighty years. Now. If you don’t mind, Dresden, might we proceed?”
“To do what?” I asked.
“To teach you what are obviously badly needed lessons,” Sir Stuart said, “and to stop these pirates.” He spat the last word with a startling amount of venom.
I frowned and eyed the gunmen, who were all reloading, having emptied their weapons in sheer, nervous excitement. They weren’t particularly good at reloading, either.
“Hell, one man with a handgun could take them all right now,” I said. “Too bad neither of us has one.”
“We cannot touch flesh,” Sir Stuart said. “And while it is possible for a shade to, for example, move an object, it is impractical. With practice, you could push a penny across a table over the course of a couple of minutes.”
“Too bad neither of us has a penny,” I said.
He ignored me entirely. “That’s because we can put forth only minuscule physical force. You couldn’t lift the coin into the air against the pull of gravity.”
I frowned. This sounded a lot like a basic lesson most young wizards received. Most of the time, when you wanted to move something around, you didn’t have the kind of energy you needed stored inside you. That didn’t mean you couldn’t move it, though. It just meant you had to get the energy to do so from another source. “But . . . you can co-opt energy from elsewhere?”
The big man pointed an index finger at me, a smile stretching his mouth. “Excellent. We cannot interact with something being moved by a living creature. We can’t even touch an object that is being carried too closely to a living body. But . . .” He glanced up at me, inviting me to finish the thought.

Ghost Story Chapter 12, Page 130-131