A fun experiment that can teach proportions, among numerous things, is calculating the growth rate of your student’s fingernails over the course of the year using data from one week.
Monday: Have your students measure their thumbnail from root to tip (or other fingers, but beware of the one they will likely pick :) with as accurate of a measuring device as you have access to. Have the students record their information on some type of lab sheet or worksheet. Between now and then you might want to have your students ask their science teacher what fingernails are made of and why they grow.
**Note, this is a great exercise to start off the class on Monday's when everyone is sleepy and cranky**
The following Monday have your students re-measure their fingernail and calculate the growth as accurately as possible. Next, have your students contemplate how they would use this data to figure out how long their fingernails will be in 6 months, 1 year, 5 years, 10 years, and 100years (you may want to provide them with an equation or a proportion)
Finally, use the picture below and ask your students to estimate, based on their data, how long it took this women to let her fingernails grow