Everything needs to be clean, so that the surface tension of the water can hold the boat and then be broken down by the soap.
Another experiment, which looks particularly impressive if colours are added, is to fill a glass up to the brim with water and then keep dropping water very carefully in so that you can see the curved surface of the water higher than the glass, held there by surface tension. When one drop of washing-up liquid is added it reduces the surface tension and the water streams over the edge in a mini waterfall.
Who did early work on surface tension? A letter to Nature published on March 12 1891 (www.physics. ucla.educwparticlespockelspockels.html) reveals that Agnes Pockels (1862-1935), a young German woman caring for relatives and isolated from the scientists of the time, experimented at home before sending her work to Lord Rayleigh, president of the Royal Society 1905-008, who won the Nobel prize for his work on the density of gases. Discuss whether great progress in science could happen in this way now. Why haven't we heard of Pockels? Find out about other women who did significant work in the wings of science, for example, Caroline Herschel.