1. The problem statement, all variables and given/known data The question asked is: Two students are given 2 stones and a waterproof stopwatch, along with swimwear. Using this material they are then required to estimate the distance across a lake. This question could obviously be answered using fairly simple process, however, it is posed in a fluid mechanics subject and therefore I assume must in some way involve the water in the lake. 2. The attempt at a solution At this point in time my only attempt at solving this question would be to one person on either side of the lake. Have one person slam the rocks together underwater and to time the response. However, i don't think this is a very good solutions. Any help would be great, thanks!