Knots are interesting objects to experiment since their topological properties are combined with mechanical properties, fluid mechanics or dynamical properties like unknotting under the excitation of the thermal energy. Knots play also a role in biology because DNA is often knotted inside the cell's nucleus and during cell division entanglements of DNA strands are an obstacle for cell division. Moreover, a large number of cancer drugs target topoisomerases, the latter being a class of enzymes that can change the DNA topology as needed by the complex machinery of gene expression or of cell division. Here we present experiments that can be easily performed with little hardware and cost that were stimulated by the article of Pieranski et al. [New J. Phys. 3, 10 (2001)].