Teenage striker Tobi Adeyemo was both stunned and elated to bag a Watford goal on his league debut.
The 17-year-old substitute struck four minutes after coming on in the 68th minute, with Ismaila Sarr's late penalty then sealing victory.
The win lifted Slaven Bilic's injury-hit Hornets into third place with Blackpool, win-less in the league since October, dropping to second bottom.
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