'Grey' aliens appear like that because they've taken DNA from humans they have abducted, zany Professor David Jacobs claims.
Giving his unusual theory on grey aliens, the professor of history at Temple University specialising in Ufology, claims it all 'boils down' to them having a slit for a mouth.
After highlighting the similarities in appearance between grey aliens and humans, the professor is convinced they've been created from the DNA of people that have been abducted.