Let f : {−1, 1} n → R be a real function on the hypercube, given by its discrete Fourier expansion, or, equivalently, represented as a multilinear polynomial. We say that it is Boolean if its image is in {−1, 1}.We show that every function on the hypercube with a sparse Fourier expansion must either be Boolean or far from Boolean. In particular, we show that a multilinear polynomial with at most k terms must either be Boolean, or output values different than −1 or 1 for a fraction of at least 2/(k + 2) 2 of its domain.It follows that given oracle access to f , together with the guarantee that its representation as a multilinear polynomial has at most k terms, one can test Booleanity using O(k 2 ) queries. We show an Ω(k) queries lower bound for this problem.Our proof crucially uses Hirschman's entropic version of Heisenberg's uncertainty principle.