Don’t do this (from an 1850 report in a medical journal):
A bottle in which some potassium had been kept in naphtha, and which had been used up in experiments, was standing in his room; and wishing to urinate without leaving his room, he pulled out the glass stopper and applied his penis to its mouth. The first jet of urine was followed by an explosive sound and flash of fire, and quick as thought the penis was drawn into the bottle with a force and tenacity which held it as firmly as if in a vice. The burning of the potassium created a vacuum instantaneously, and the soft yielding tissue of the penis effectually excluding the air, the bottle acted like a huge cupping glass to this novel portion of the system. The small size of the mouth of the bottle compressed the veins, while the arteries continued to pour their blood into the glans, prepuce, etc. From this cause, and the rarefied air in the bottle, the parts swelled and puffed up to an enormous size.