How I know aliens like blackberries

George and I were walking over the tip today when a man came towards us. He was dressed in that sort of cover-all uniform that people wear if they have to be in contact with someone with a contagious disease - or aliens. He nodded a greeting and then added, 'I'm just zapping knotweed.'
'Okay.'

We walked on and after a moment George said, 'Are you thinking what I'm thinking?'
'That there isn't any knotweed in this area?'
'Yep.'
'But why would he lie to us?'
George looked over one shoulder and the the other before whispering, 'Aliens.'
'ALIENS!'
'Sshhhhhhhh!'
'But there's no sign of alien life.'
'Precisely.' George nodded wisely. 'They've cleared it all up so the public don't start to panic.'
I nodded. 'That makes sense.'

We walked on a bit further and then I said, 'That would explain why there were no blackberries on that bit of the path too.'
George frowned. 'Why?'
'The aliens must have picked them before they were discovered.'
George laughed, 'Now that's a bit far-fetched, don't you think?'