The two-dimensional material graphene is often depicted as a hexagonal mesh of carbon atoms, with plenty of space between its atoms. But in reality, the finite size of the carbon atoms leaves little room for anything to slip through. In 2008, a classic experiment1revealed that pristine graphene is impermeable to helium and other gases at room temperature, making it the thinnest barrier known to science. The results logically extend to other two-dimensional materials, including hexagonal boron nitride (hBN) and molybdenum disulphide (MoS2). By contrast, in a paper published in this issue (page 227), Hu et al.2 present the unexpected finding that graphene and hBN — but not MoS2 — are excellent conductors of protons across their two-dimensional structure.