Children not only enjoy discovering that teachers are fallible but that their treasured calculator can do silly things.
If you have a super-duper calculator, this won't work! But many of the calulators we have in schools do not remember beyond six decimal places. So try this:
1 / 7 x 7 = some calculators give the expected answer of '1', but others will return '0.9999997' or something similar.
This simple exercise can lead naturally on to the discovery of recurring numbers and the particularly interesting recurring decimal approximation of 1/ 7.
This 6 figure number (.142857) has 'magical' properties as most year 4+ pupils will discover. Try multiplying it by 2 or 3 or 4 or 5 etc and look at the result. They will be intrigued!