That my pup can't wriggle out of! We currently have a Pets at Home one but it's rubbish. It loosens off as we're out and about and if she's pulling away from something (she hates going in the car for instance), she can get out of it! On Christmas Day I had my toddler running in one direction and pup in the other. Luckily pups recall is 100% better than ds's.