  We are given an acyclic directed graph with one source, and a subset of its
edges which contains exactly one outgoing edge for every non-sink vertex. These
edges determine a unique path from the source to a sink. We can think of it as
a switch in every vertex, which determines which way the water arriving to that
vertex flows further. We are interested in determining either the sink the flow
arrives, or the whole path, with as few questions as possible. The questions we
can ask correspond to the vertices of the graph, and the answer describes the
switch, i.e. tells which outgoing edge is in our given subset. Originally the
problem was proposed by Soren Riis (who posed the question for pyramid graphs)
in the following more general form. We are given a natural number k, and k
questions can be asked in a round. The goal is to minimize the number of
rounds. We completely solve this problem for complete t-ary trees. Also, for
pyramid graphs we present some non-trivial partial results.
