Your granny normally took a hot 2L hot water bottle (covered with a thin towel and at 50 degrees Celsius) with her to bed on a very cold winters night and it worked quite well. She however decided to replace the water bottle with a huge 2 kg piece of iron (also covered with a towel at 50 degrees Celsius). She only used the 2kg piece of iron for one day and decided to rather always use the hot water bottle throughout the cold winter. Use your knowledge of Physics to explain why granny only used the hot piece of iron for ONE day and went back to using the hot water bottle aferwards.

Your granny normally took a hot 2L hot water bottle (covered with a thin towel and at 50 degrees Celsius) with her to bed on a very cold winters night and it worked quite well. She however decided to replace the water bottle with a huge 2 kg piece of iron (also covered with a towel at 50 degrees Celsius). She only used the 2kg piece of iron for one day and decided to rather always use the hot water bottle throughout the cold winter. Use your knowledge of Physics to explain why granny only used the hot piece of iron for ONE day and went back to using the hot water bottle aferwards.