function hungry(uint256 _tokenId) public {
	require (piranhaIdToOwner[_tokenId] == msg.sender);

	Piranha storage piranha = piranhas[_tokenId];
	require (piranha.hungry == 0);
	
	uint8 piranhaSize=uint8(piranha.size+(now-piranha.growthStartTime)/900);

	require (piranhaSize < 240);
	
	piranha.hungry = 1;
  }