This is a fun activity that I usually reward the winner (or top 5 best) with house points. Basically, every student goes off with the worksheet and estimates (in cm) the length of each line using ONLY their eyes and brains. Once that's done, everyone goes back and measures them properly using rulers. Then the whole class sits with the teacher with the spreadsheet provided and they see their estimates entered and see how their score (reflected by how accurate their guess was) decrease. The student with the lowest score (ie: the most accurate) wins!
* PDF with teacher notes, answer key and student worksheet
* Excel Spreadsheet (Excel 2013 workbook, but can make a compatible one by request)