It’s almost summer, which means it’s nearly the season for the time-honored tradition of playing in fire hydrants on a hot day. And while it can be great fun to direct the spray with a bottomless coffee can or even drill holes in the valve cap to make a makeshift sprinkler, fire hydrants emit high-velocity streams of water, making it a really bad idea to jump in front of all that water.

As a prime demonstration of that danger, a video of a worst-case scenario made it toward the top of the r/WTF subreddit this week. The video comes from Lad Bible’s Facebook page, where it racked up over 4 million views. In the video, titled “What not to do with a open fire hydrant,” a man shows us exactly what not to do with an open fire hydrant. In the video, a group of people, seemingly coming from a bar at night, cheers as one man runs toward the water streaming up from the sidewalk.

While it may seem obvious that you shouldn’t jump into all that dang water, the full consequences of this man’s actions still seem startling. Let’s look at the physics of fire hydrants to see why this happened.

Most municipal fire hydrants discharge water at between 50 and 100 pounds per square inch of pressure. Perhaps more importantly, though, they flow massive amounts of water. Hydrants can flow anywhere from 500 up to 1,500 gallons per minute. At the high end, that means 25 gallons of water are coming out every second. So in a split second, many gallons, and therefore pounds, of water are coming out at a really high pressure. This can cut your skin or, as shown in this video, make you bust your ass. And the wider open the fire hydrant’s flow valve is, the higher the pressure and flow are.

When people illegally open fire hydrants, they often just crack the valve, which lets out a relatively small amount of water at a relatively low pressure. As you open the valve, though, the velocity of the water increas