My math challenge in the 6th grade social studies class I teach involved estimating this time around.

Given that there are almost 70 standard miles in a degree of latitude, show *how* you can estimate the circumference of the earth.

But I guess I'm forcing them to estimate by not telling them that there are 69.1 or so standard miles in a degree of latitude. I also drew a big circle on it to encourage them to use a picture.