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I am a regular traveller between the Scottish island where I live and the mainland. The vibration on the ferry sets up an interesting interference pattern on the surface of a cup of coffee. It forms quickly after the cup is put down then remains fixed. Presumably, waves are reflected from the sides of the cup and subsequent waves reinforce the earlier ones. The pattern remains as the coffee level gets lower. In the interests of research, perhaps next time I should investigate whether the effect occurs in a glass of beer! An explanation would be appreciated.

Jack Harrison, Tobermory, Isle of Mull, UK

This article appeared in print under the headline “Caffeine Shakes”