Taking a piece of skin from the belly and switching it with a piece from the back caused the frog to scratch its belly when you tickle its back.
'What Sperry was demonstrating here was evidence of what has become known as the Chemoaffinity hypothesis. When the axons (the ‘wires’ of the nerve cells) are developing they are predetermined by their genes to seek out specific cells by attaching to a particular chemical signature. So when Sperry switched the location of the back and belly skin cells the axons still found the tissue they were intended to find. Hence, the tickling of the switched around areas fooled the frog into believing the irritation was at another location of its body.'