Gordon R. Dickson's Danger — Human.
From what I can recall of the plot of the story it is about this one man in a jail cell in the middle of a huge room that is supposed to be impossible to get out of. He is surrounded by a moat of some sort and two guards.
They marched to the door of this building and it opened without any of them touching it. Inside was perhaps twenty feet of floor, stretching inward as a run inside the walls. Then a sort of moat—Eldridge could not see its depth—filled with a dark fluid with a faint, sharp odor. This was perhaps another twenty feet wide and enclosed a small, flat island perhaps fifteen feet by fifteen feet, almost wholly taken up by a cage whose walls and ceiling appeared to be made of metal bars as thick as a man's thumb and spaced about six inches apart. Two more of the aliens, wearing a sort of harness and holding a short, black tube apiece, stood on the ledge of the outer rim. A temporary bridge had been laid across the moat, protruding through the open door of the cage.
He escapes from this seemingly impossible cell eventually by repeatedly throwing up and using his stomach acid to slowly weaken the bars of his cell.
"The hinges of the hatch," he said, "were rotten—eaten away by acid."
"Acid?" the commander stared at him. "Where would he get acid?"
"From his own digestive processes—regurgitated and spat directly into the hinges. He secreted hydrochloric acid among other things. Not too powerful—but over a period of time—"
There was also some sort of shield that was supposed to be unpassable.
"Hasn't he?" said the doctor thickly. "What about the defensive screen—our most dangerous most terrible weapon—that could burn him to nothingness if he touched it?"
The commander stared at him.
"But—" said the commander. "The screen was shut off, of course, to let the food carrier out, at the same time the door was opened. I assumed—"
"I checked," said the doctor, his eyes burning on the commander. "They turned it on again before he could get out."