Summarize this article in one sentence.
. An anemometer is a tool for measuring wind speed. You can buy one online, or make a simple one yourself in about 30 minutes using the steps below. If you bought one, skip down to the step in which you count the rotations – or just read the wind speed directly, if yours has a digital display. Take four small paper cups, and punch a single hole in each one, about ½ inch (1.25 cm) below the rim. Take a fifth cup, and puncture four evenly spaced holes, about ¼ in. (6 mm) below the rim, then punch a fifth hole in the center of its base. You can use a pencil to punch these holes, if you don't have anything sharper. Stick a plastic straw into a single-hole cup, about 1 inch (2.5 cm) into it. Push the other end of the straw through two holes of the five-hole cup. Stick the free end of the straw into another single-hole cup. Turn the two single-hole cups so they are pointing in opposite directions, along the same plane as the straw. Staple the straw to the cup. Repeat with another straw, putting it through the remaining two holes of the central five-hole cup. Rotate these two new cups until each cup opening is nearer the base of the next one. In other words, the top cup points right, the right cup points down, the bottom cup points left, and the left cup points up. Staple the straws and cups together. Slide the two straws until all four cups are the same distance from the center. Stick a small pin through the intersection of the two straws. Stick the eraser end of a pencil through the hole in the base of the central cup, and push it gently onto the pin. You can now hold the anemometer by the pencil tip, and use it to measure wind speeds. Hold the anemometer upright in a windy area. Watch a single cup (draw on it with a marker if it makes it easier to follow) and count the number of times it rotates. Use a stopwatch or a friend watching the second hand of a clock to time 15 seconds, and stop you when the time is up. Multiply your count by four to get the number of revolutions per minute (rpm). For greater accuracy, count the number of rotations in 60 seconds (and don't multiply by anything). Measure the distance from one edge of the anemometer to the other to find the diameter of the rotating circle, d. The circumference of the circle, is equal to πd. This is the distance traveled in one revolution. If you don't have a calculator, you can use 3.14 as an estimate of π, or even just 3 for a rough estimate. Convert the circumference you calculated into a more useful unit for measuring wind speed (miles or kilometers). Multiply the result by the rpm you calculated to get the total distance traveled in one minute. Multiply the result by 60 to get the distance traveled in one hour (mph or km/h). Here are the full formulas in imperial and metric units:  Imperial: (__circumference__ inches/revolution) * (1/12 ft/inches) * (1/5280 miles/ft) * (__rpm__ revolutions/minute) * (60 minutes/hour) = __wind speed__ in miles per hour. Metric: (__circumference__ centimeters/revolution) * (1/100000 kilometers/centimeter) * (__rpm__ revolutions/minute) * (60 minutes/hour) = __wind speed__ in kilometers per hour.

Summary:
Decide whether to make or buy an anemometer Punch holes in small paper cups. Construct one half of the basic shape. Finish the basic shape. Make a base for the anemometer. Count the number of rotations the anemometer makes. Calculate the circumference. Calculate wind speed.