I've just spend about an hour fishing bits of old cat turd out of my boys' sandpit and I'm getting fed up with it. Any tips on keeping them out gratefully received. We've tried making a lid but the first (designed by DP) failed to keep them out (like I told him it would ) and the second, designed by me, worked till DP 'accidentally' broke it. the sandpit is quite big (2sq m) so difficult to cover. Any ideas?