"Una furtiva lagrima" from Donizetti's L'elisir d'amore
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Andrea Bocelli: Opera - an album guide

10. "Una furtiva lagrima" from Donizetti's L'elisir d'amore
Donizetti is one of the masters of opera, and it's easy to see why here. The narrator, Nemorino (or Andrea Bocelli, as you may know him), gets gradually more excited as he realises a love potion has had the desired effect on the object of his affection.