Cally read out the next instruction: 2. Make a cut along the midline of the belly from the pelvis to the throat. Be careful to cut only through the skin. Avoid damaging the organs beneath. I used tweezers to catch and lift the skin, punctured it with the scissors and cut a straight line all the way up the body. It was thinner than I expected, though I could feel the slight resistance of tissue. I imagined the living thing it had been, the bulging pulse of its neck, the slow closing and opening of its eyes.

Cally had been angry that we had to use a real frog, arguing in that intense way of hers about animal rights: what could we learn from cutting open a dead frog that we couldn’t learn from a book? ‘It’s only an animal,’ I had said, and laughed, just to goad her. ‘It’s dead now, anyway.’ And I was curious.