Seth’s Pond in West Tisbury froze over last week and after a light snowfall a small band of hardy Islanders got out the shovels and cleared the way for a little pond hockey, kids and dogs included. A similar scene played out on Duarte’s Pond, a few clicks down Lambert’s Cove Road. When freshwater ponds freeze, for diehard skaters there is not a moment to waste. Because today’s frozen pond can be tomorrow’s open water for ducks wintering over. And that is the problem. Frozen ponds can be treacherous, especially for children and dogs who are innocent of the dangers posed by thin ice. Saltwater ice is even more unstable than freshwater ice. So the rule is simple — don’t go out on a frozen pond unless it has been declared safe for skating by a responsible adult, such as a member of the town fire department. And if you see a person or a dog in trouble on the ice, call 911.