National Weather Service United States Department of Commerce

Learning Lesson: Going with the Flow


Bernoulli's principle states that in fluid flow, an increase in velocity occurs simultaneously with decrease in pressure. The students will discover that the faster air moves (air acting as a fluid), the lower the pressure becomes within that flow of air. They will see this effect blowing between two soda cans.

TOTAL TIME 5 minutes
SUPPLIES Two (2) empty soda cans, a level surface
SAFETY FOCUS Tornado safety


  1. Lay the two cans parallel to each other, about one inch apart, near the edge of a level surface.
  2. Put your face down near the surface and blow lengthwise between the two cans.
  3. It will take some trial and effort but eventually the two cans will roll together.
  4. Another way of demonstration is by suspending two cans with string about an inch apart and having the student blow between them.


The affect is Bernoulli's principle in action, named after the Dutch/Swiss mathematician/scientist Daniel Bernoulli. By blowing between the two cans, you are making the air between them move faster than the surrounding air (which is basically calm). The cans roll together as the higher pressure surrounding the two cans (away from the air flow) pushes the cans together toward the region of lower pressure.

Building a Weather-Ready Nation

There is no such thing as guaranteed safety from a tornado. Freak accidents happen; and the most violent EF5 tornadoes can level and blow away almost any house and its occupants. Fortunately, most tornadoes are much weaker and can be survived.

At home, have a family tornado plan in place, based on the kind of dwelling in which you live. Know where your safe rooms are and practice a family tornado drill at least once a year. Have a pre-determined place to meet after a disaster.

Flying debris is the greatest danger in tornadoes; so store protective coverings (e.g., mattress, sleeping bags, thick blankets, etc.) in or next to your shelter space, ready to use on a few seconds' notice. When a tornado watch is issued, think about the drill and check to make sure all your safety supplies are handy. Turn on local TV, radio or NOAA Weather Radio and stay alert for warnings.

Forget about the old notion of opening windows to equalize pressure; the tornado will blast open the windows for you! If you are out shopping, know the locations the store's bathrooms, storage rooms or other interior shelter areas away from windows, and the quickest way to get there should a tornado strike.

All administrators of schools, shopping centers, nursing homes, hospitals, sports arenas, stadiums, mobile home communities and offices should have a tornado safety plan in place, with easy-to-read signs posted to direct everyone to a safe, close-by shelter area.

Schools and office building managers should regularly run well-coordinated drills. If you are planning to build a house, especially east of the Rockies, consider an underground tornado shelter or an interior "safe room".