We have studied the folding mechanism of $eta$ -hairpins in the proteins 1GB1, 3AIT and 1A2P by conducting unfolding simulations at moderately high temperatures. The analysis of trajectories obtained from molecular dynamics simulations in explicit aqueous solution suggests that the positions of the hydrophobic core residues lead to subtle differences in the details of folding dynamics. However, the folding of three different hairpins can be explained by a unified mechanism that is a blend of the hydrogen-bond-centric and the hydrophobiccentric models. The initial stage of $eta$-hairpin folding involves various partially folded intermediate structures which are stabilized by both the van der Waals interactions of hydrophobic core residues and the electrostatic interactions of non-native hydrogen bonds. The native structure is obtained by forming native contacts in the final tune-up process. Depending on the relative positions of the hydrophobic residues, the actual mechanism of hairpi n folding may or may not exhibit well-defined intermediates.