Phase separation involving biomolecular liquids is emerging as a fundamental mechanism of intracellular organization. Liquid-liquid phase separation (LLPS) compartmentalizes cellular contents into various biomolecular condensates to facilitate spatio-temporal regulation of biological processes. In this talk, I will discuss our recent efforts to understand the organization and function of biomolecular condensates. In the first half of my talk, I will share our recent work where we take a bottom-up approach to build programmable synthetic condensates. Taking advantage of the remarkable self-assembly behaviors of DNA, we demonstrate that the composition and function of DNA-based condensates can be fine-tuned. Our synthetic condensates illustrate how the collective interactions of condensate components can give rise to emergent functionalities. In the second half of my talk, I will discuss the biomolecular density of condensates in living cells. As a fundamental physical property, the density represents one of key factors influencing the way biomolecules interact with one another. We find that a class of condensates has surprisingly low biomolecular densities, and that RNA plays an important role in defining the density of condensates.