Emalyn raised her hand, her heart sinking (though feeling oddly as if it were rising toward her throat...) "A boggart, Proffessor. They feed on fear, but are weak when laughter or happiness is involved. It's also similar with dementors, though I think dementors would carry on even if you were roaring with laughter."
Ema found herself looking cautiously around her, where possibly boggarts could be enclosed. Oh great, she said, fighting the urge not to groan.