The Eers are made of rubber that's quite firm, thanks to the hardened silicone filling, so even though they're molded to the shape of your ears, they're kind of stiff and inflexible. That leads to small fitting problem: The earbuds need a slight amount of liquid to form a quality seal in the ear. Sonomax gave me a tiny bottle of ear-lube, which has proved damned difficult to explain to strangers, so I usually leave it at home. Water works just fine, though, as does (not to be gross, but I'm about to, so) sweat, which is partly why they're great exercise 'buds. Natural lubricant! Disgusting! Effective!