This male Greylag approached me nervously in the park today. It hissed regularly while walking towards me, probably expecting handouts. I crouched down and tried to get a close shot of its beak as it hissed. Geese - and the duck family, Anatidae, in general - have very distinctive beaks with a rounded, 'nail' at the top tip (young Greylags have a black nail) and serrated edges alongside both sides of the upper and lower bill. In the case of the geese, these are quite hard and sharp, remarkably teeth-like (they can draw blood if they get to bite your finger). When looking at the photo I noticed even the tongue has serrated projections. In geese, these help cut grass and plant material when they are grazing. In some ducks these edges have wafer-thin lamellar forming something akin to a whale's baleens, helping them filter out small planktonic organisms from the water.