  Fix integers $n \ge r \ge 2$. A clique partition of ${[n] \choose r}$ is a
collection of proper subsets $A_1, A_2, ..., A_t \subset [n]$ such that
$\bigcup_i{A_i \choose r}$ is a partition of ${[n] \choose r}$. Clique
partitions are related to design theory, coding theory, projective geometry,
and extremal combinatorics.
  Let $\cp(n,r)$ denote the minimum size of a clique partition of
  ${[n] \choose r}$.
  A classical theorem of de Bruijn and Erd\H os states that $\cp(n, 2) = n$ and
also determines the extremal configurations. In this paper we study $\cp(n,r)$,
and show in general that for each fixed $r \geq 3$, \[\cp(n,r) \geq (1 +
o(1))n^{r/2} \quad \quad {as}n \to \infty.\] We conjecture $\cp(n,r) = (1 +
o(1))n^{r/2}$, and prove this conjecture in a very strong sense for $r = 3$ by
giving a characterization of optimal clique partitions of ${[n] \choose 3}$ for
infinitely many $n$. Precisely, when $n = q^2 + 1$ and $q$ is a prime power, we
show \[ \cp(n,3) = n\sqrt{n-1} \] and characterize those clique partitions
achieving equality. We also give an absolute lower bound $\cp(n,r) \geq {n
\choose r}/{q + r - 1 \choose r}$ when $n = q^2 + q + r - 1$, and for each $r$
characterize the finitely many configurations achieving equality with the lower
bound. Finally we note the connection of $\cp(n,r)$ to extremal graph theory,
and determine some new asymptotically sharp bounds for the Zarankiewicz
problem.
