What country originated the tradition of placing candles in windows to welcome holiday visitors?

Ireland

Which marine creature famously clings to whales and sharks, feeding on their parasites and scraps? Which animal routinely collects and utilizes specific plants to self-medicate illness? What are you required to yell when you have just one card remaining in the card game Uno? Which scientist is known for the laws of planetary motion? What does the identity matrix look like?