Cannizzaro reaction

The Cannizzaro reaction takes place between carbonyl compounds without a hydrogen that can be abstracted by a strong base. Common examples are benzaldehyde and formaldehyde.
The latter is used in the animation below.
The OH^- ion acts as a nucleophile and attacks the carbonyl carbon, to form a tetrahedral intermediate.

The most likely reaction of this intermediate is the reverse process: expulsion of hydroxide.
The hydride (H^-) is a less likely leaving group. Still, a hydride shift is observed when there is a second formaldehyde in the neighbourhood, to act as a hydride acceptor.

The animation shows the subsequent steps, starting with hydroxide attack.

From right to left we have two OH^-'s, two formaldehydes and a water molecule.

hydroxide attacks formaldehyde

(Un)colour hydrogen that will move as hydride.

The products of the hydride shift are formic acid and the methanolate anion. Under the reaction conditions these two will rapidly loose and gain a proton, respectively, making the overall reaction irreversible.
This way, the reaction will be driven to completion.