Tautomerization

The tautomerization of aldehyde and ketones is catalyzed by small amounts of acid or base.

Acid Catalyzed Tautomerization

Under acid catalysis, the carbonyl is first protonated.  The protonated carbonyl is really just a resonance stabilized carbocation as seen in its resonance structure.  The carbocation is deprotonated to form the enol.

These steps are reversible so if you treated a ketone with acidic D₂O, all the α protons would eventually exchange with the D atoms of the ketone or aldehyde.  

Base Catalyzed Tautomerization

In base catalyzed tautomerization, a resonance stabilized enolate intermediate is formed.  Protonation of the enolate O atom results in the enol and regenerates a hydroxide ion (^-OH) .