Unlike p- and d-orbitals, s-orbitals do not have a node (i.e. a region of zero electron density) at the nucleus. In fact s-orbital electron density actually penetrates right into the atomic nucleus. The amount of s-electron density involved is tiny but enough to produce observable effects. One significance of this effect is that s-electrons - or electrons in orbitals with a high % s-character - are attracted to and stabilised by the positively charged nucleus more strongly than electrons in orbitals with little or no s-character. This, in turn, influences the acidity of C-H bonds.