A bee or a fly?

Kate and Mick sat in the field and unpacked the picnic hamper. Out came the sandwiches, cakes, vacuum flask, plates, cutlery. During the whole unpacking procedure, Kate was troubled by an insect constantly buzzing around her face. Despite many swipes of her hand, the little blighter would not be driven away.

'In the name of God, Mick,' she squealed, 'what is it, a bee?'

'No,' said Mick. 'It's a dum dum fly. They hang around the back end of cows.'

'What?' screamed Kate. 'Are you trying to say my face is like the back end of a cow?'

'No,' said Mick. 'But you'll have to convince that fly!'