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How to walk your hamster
Choose a safe, snug harness.
There are plenty of small animal harnesses available at your local pet store or online. It may be worth visiting the store first to familiarize yourself with the different options and sizes that are available.
You'll likely need the smallest harness you can find, as animals the size of hamsters are not commonly walked. Go with a harness that fits snugly, without impeding the hamster's breathing or movement.