One of the most common reactions of an aldehyde or ketone is the attack of a electron-rich species on the electron-defficient carbonyl carbon.
Lowest Unoccupied Molecular Orbital (LUMO) of formaldehyde, ready to accept electrons from the Highest Occupied Molecular Orbital (HOMO) of an electron-rich species. The attack will take place on carbon which bears the orbital highest coefficient.
Water addition to formaldehyde
Formaldehyde's carbon, initially planar with sp2 hybridization, suffers the attack of the electron cloud of water's oxygen. The reaction proceeds through a TS where the carbon turns into a tetrahedron, swapping its hybridization from sp2 to sp3.
The nucleophiles able to add to a carbonyl are quite diverse, with different synthetic consequences.
Nucleophilic Addition to Carbonyl
Reactions of Enols and Enolates