  We show that for every positive epsilon there exist positive delta and n_0
such that every 3-uniform hypergraph on n>=n_0 vertices with the property that
every k-vertex subset, where k>=delta*n, induces at least (1/4 + epsilon)*{k
\choose 3} edges, contains K4- as a subgraph, where K4- is the 3-uniform
hypergraph on 4 vertices with 3 edges. This question was originally raised by
Erdos and Sos. The constant 1/4 is the best possible.
