In the German language, the word "Eis" is colloquially used for both "ice cream" and "frozen water". When someone uses that word, figuring out what exactly she refers to requires context. Spelled and pronounced identically, "Eis" is a [homonym][1] (technically, a polyseme, explained in the same Wikipedia article).

If you're standing at a lake in winter, context suggests that you're referring to frozen water: "Eis wäre jetzt schön" would be understood as a wish for the lake to be frozen (roughly "it would be nice if there was ice on the lake").

If you're standing at a swimming pool in summer, context suggests you're referring to ice-cream: "Eis wäre jetzt schön" would be understood to mean "some ice-cream would be nice right now".

There are scenarios where there is no implicit context, as described in other answers. In a supermarket that sells both ice-cream and ice cubes, for example, asking "wo haben Sie Eis?" would prompt the question "which kind, cubes or ice-cream?" or, if the clerk is distracted, likely cause the obvious misunderstanding.

If context is unavailable, a more specific term than "Eis" must be used to prevent ambiguity.

  [1]: https://en.wikipedia.org/wiki/Homonym