Estimation 180 is a hit
I'm late to the party on this one, but Andrew Stadel's Estimation 180 is a pretty cool tool. Today in the Summer Enrichment Program (rising 8th graders, all boys), I went through Days 1 through 4. It was all height-related. In each one, I provided a worksheet where the students would make their guess and give a reason why. I took some random responses and got a class high and low for each prompt.
In the process, students brought up the ideas of comparing to other objects, ratio, inventing new units (a Stadel and a Stadel Head were both mentioned in each of the 4 classes without any prompting), what "tall" means, perspective, and depth of field. In general, we discussed how modeling is messy, with people slouching, wearing different types of shoes (and how it impacts height), and standing on different levels from comparative objects. Gosh it was fun. And they agreed. Don't trust me, listen to this, the reaction in one class after revealing the Day 4 solution (SPOILERS):
Those are 8th graders. In the summer. Doing math.
Next time I do some of these, I'll add a confidence indicator to their response sheets.