Helen, you can also think about the displacement of mass. Ice floats because it is less dense than liquid water (because water expands as it freezes), but it will, like anything, displace its mass. That's why the level didn't change in your experiment. So, ice on land doesn't even have to melt to raise sea levels, it just has to be moved into the sea. For instance if a chunk of ice broke off of Greenland or Antarctica (maybe because melted water running beneath it had caused a void) and fell into the sea, the sea level would rise, just as if you'd dropped another ice cube into your measuring cup.