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I guess, tea works fine for several good reasons: It is usually available without problems, you can drink it after the shots (while unboiled pure water might be risky), and finally, tea contains aromatic oils - they often leave brown traces on a white cup. These oils likely form a one mol thin film on the water, calming its surface. With pure water, the lightest air breeze causes a scattered sight, and you have to wait for seconds until the waves disappear.