There are many types of touch-sensitive nerves in your skin. Some nerves respond to pressure, some to fast vibrations, some only to slow vibrations. Everything you touch—water, a rubber ball, a feather—stimulates a distinct combination of nerves that helps you identify what you’re touching.
Here, there’s clearly something special about the combination of nerves stimulated, creating what many people describe as a slippery or velvety sensation. But no one knows exactly why this happens.
This strange phenomenon was first noticed by a staff member when she was a kid, idly stroking a playground fence.