  We give an explicit construct of a harmonic weak Maass form $F_{\Theta}$ that
is a "lift" of $\Theta^3$, where $\Theta$ is the classical Jacobi theta
function. Just as the Fourier coefficients of $\Theta^3$ are related to class
numbers of imaginary quadratic fields, the Fourier coefficients of the
"holomorphic part" of $F_{\Theta}$ are associated to class numbers of real
quadratic fields.
