The law says once a year zoo keepers must check the number of animals they think they have is correct, to keep an eye on breeding and to make sure none have gone walkabout.
London zoo keepers have a big job on their hands making notes of the tens of thousands of creatures in their care. This Silvery Marmoset monkey is wondering what's going on!
This animal, called a sloth, which lives in the zoo's rainforest, is certainly enjoying the attention!
And this African Black Footed Penguin is also curious to know what the fuss is about
But not all of the animals are willing to keep still and be counted like this owl butterfly.