Pi is an interesting mathematical idea for young kids. The thought that a measurement can be same, everywhere in the world, every time, is a hard concept to grasp. I came across this activity in the great Teaching Student Centered Mathematics by John van de Walle.
1. Ask the students to measure the Circumference and Diameter of 5 circles in and around the classroom. Try and get a variety of sizes of circles.
2. Make a Table to record your work
3. Once finished measuring, find the ratio of Circumference to Diameter by dividing C by D. Add this information to your table.
4. Take you data and record it on a large, class sized Scatter Plot.
5. Analyze data from Scatter Plot and Tables; look for conclusions or patterns
This should lead into a discussion about why all of the numbers for the ratios are close to 3.14. Once we see this, you can introduce the concept of Pi. From there, this lesson could fractal out into so many different dimensions and independent inquiries.
To further excite their curiosity, here is a website that shows Pi to four million places. Another little tidbit of information, a team at Tokyo University have calculated Pi to 1.24 Trillion places!