James O'Brien is Leading Britain's Conversation.
On the roof of some vans, you get black spinning discs. What are they?
** Definitive **
Name: James, Westminster
Qualification: We drive guard dogs around
Answer: They are called “fletner vents” on vans and are used when you are transferring dogs or food. As there are no windows in vans, there needs to be another way to circulate fresh air – and also get rid of the smell.