This came after we've spent a fair bit of time on Andrew Stadel's great site estimation180.com, recording our upper and lower limits and estimation on the Google docs for some of the challenges. There's something really engaging about being presented with these mystery quantities, amounts, numbers and measurements, and trying to zero in on a reasonably accurate answer.
The Year 4 students produced a real variety of challenges: measuring ones like how tall is Milly, the world's smallest dog?
how many matches are left in the matchbox?
And lots of counting challenges, including some where you need to sample part of the picture and then scale it up to account for the whole picture, like Hama bead estimation.
It's really great how families have helped with this, with the maths, and with filming and photographing, and also how everyone has been able to write their post, upload their pictures that give clues and also links or videos that reveal the answer. And of course the estimates in the comments have been great too! If you'd like to have a go, please do. No peeking though!