you ever taken a sip of coffee and burned your lip or tongue? Then
after you set the coffee aside to cool, it gets too cold. This lesson
is designed for students to analyze why this happens and to help
predict when to take that first sip.
In order to achieve this goal, students will take measurements of hot coffee (actually, simulated
hot coffee) at various times, and then graph the results. Before they
can determine the perfect time for the first sip, however, they must
conduct some research to determine what temperature is hot enough to
Lead a class discussion about Liebeck v. McDonald's Restaurant,
better known as the "McDonald's Coffee Case," a lawsuit regarding a
fast food restaurant and a person who was scalded by their coffee.
(Note that you may want to exclude the name of the restaurant when
discussing the case in class.) The lawyer for the defendant said that
McDonald's provided him its operations and training manual, which says
its coffee must be brewed at 195–205° and held at 180–190° for optimal
Pass out the Too Hot to Handle Activity Sheet. Tell students that they are going to determine the best
time to take the first sip of a hot drink by using a function to model
how the temperature of the drink changes over time. Ask them to predict
what kind of function will represent this relationship. If they
struggle, remind them of the kinds of functions they have studied —
linear, absolute value, quadratic, square root, exponential and
logarithmic. Do not draw the graphs, however, because they will do this
Too Hot to Handle Activity Sheet
If possible, conduct the following experiment in class. Boil a pot
of water. Then, place a thermometer in a cup and fill it with hot
water. Record the temperature about every 3 minutes. It will take
approximately 30 minutes for the coffee to cool to 120°, which is a
reasonable temperature at which the first sip might be taken.
As a time-saving alternative, you can show students the following
video that shows a mug of hot water cooling, while a thermometer and
timer indicate the temperature and elapsed time. Showing the video
gives several advantages: the data collection occurs more quickly; all
students are able to watch the thermometer throughout the entire
process; and, all students will be using the same set of data. One
interesting strategy is to require all students to collect at least ten
data points, without restriction on which data points they must choose.
Although students will collect different sets of data, the resulting
graphs should be very similar.
Using either method, students should collect data until the
temperature gets to the point where clearly no scalding will occur.
Because scalding results partially due to temperature and partially due
to the amount of time exposed to the heat, some research should be done
to find the scalding temperature. If you are doing the experiment, wait
until the temperature gets down to 120° and then gently touch the
surface of the water with your finger. If you cannot hold your finger
there for very long, then it is still too hot. Do NOT have a student do this.
If you are using the video, research can be done in class as a
teacher-led web exploration or in a lab as individual or group
research. Students should determine what temperature would be good for
a first sip. A reasonable estimate is approximately 120°.
At this point, students should graph their data. Some students will
need guidance through the setup of the axes, scales and intervals.
After plotting the data, students should choose the type of function
that best models this cooling temperatures. Finally, students should
use their scalding temperature research and their graph to determine
the best time to take the first sip.