When infants repeat actions that bring interesting or pleasurable results, primarily with people or objects in their environment, they are most likely in the sensorimotor substage.

Secondary circular reaction

Explanation: this occurs between babies of about 4-8months. Here, children acquiring pleasure from toys, object in their environment by playing with them. In this stage, children uses items or object to learn about the world. They use object by throwing them, playing with them and when they derive pleasure or happiness from playing with it, they become happy and do it continuously. In this substage, children do things and mostly expect reaction from the environment. Example they throw toys and the sound of it hitting the ground will make them happy or sad. And they will continuous do that action if they are happy with the sound it makes or find pleasure in them.

elastic potential energy

well it depends i think people around the world are coming around to gay marriage. but if you are talking abortion then wow, so much to talk about. such as men thinking they have the right to choose if women can, the fact there are health concerns if the women can't get one, and many more arguments to discuss. so i would definitely say abortion would be a much more interesting topic.