Why does a solid turn into a liquid when heat is added?

Because the molecules in heat is hot and the molecules in the solid is tight together, and when the heated molecules, come in contact with the tight molecules, it makes the tight molecules become lose and they all become freely. I hope i this help 

I'm into politics, why am i getting these questions? it's 4: 30 am in texas. my paper is due in 2 days. i'm the one needing in political science