Except in this case, as Live Science explains, that pond is our home galaxy, and the rock — our neighboring Sagittarius dwarf galaxy — is the size of 400 million Suns.
While previous studies have concluded that our galaxy indeed is rippling, this one, as Live Science explains, is the first to show that the ripples move all the way through its furthest reaches — a sign that at the time, our comparatively small neighbor was actually much bigger than it is today.