Workshop demonstrations with almost nothing: Thirty examples with a glass of water

Suppose you are suddenly assigned to a bare classroom instead of your nicely equipped physics room. What can you do to still have a clear and exciting lesson? In this workshop I present a collection of small and quick demonstrations and visualizations for secondary education, which require no equipment beyond what is present in a typical lecture room or bare classroom (Chalk, chairs, students, books, paper, student bags and typical contents). The nature of the demos is varied, some are to verify an explanation, but most are to illustrate, visualize, or simulate. This article provides 30 examples with a glass of water.