Alright, let's imagine that you have a yummy chocolate bar in your hand. When it's a solid, the chocolate is hard and you can hold it without it making a mess. But now, let's say you put the chocolate bar in a sunny spot for a while. What do you think will happen? The chocolate will start to get soft and eventually turn into a gooey, sticky liquid, right? That's because the sun makes the solid chocolate heat up and change into a liquid!
Another example is ice turning into water. When you have a big block of ice, it's a solid. But if you leave it outside on a hot day, the ice will start to melt and become water. So, solids turning into liquids is like when you have something hard and it becomes all soft and runny because it gets hot!
When things get hot enough, their particles (tiny pieces that make up everything) start moving really fast. For a solid, the particles are stuck together and can't move too much, so it stays solid. But when they start moving super fast, the particles in the solid start to spread out and move more, which changes it into a liquid.
So, to sum it up, when a solid changes into a liquid, it's because it's getting hot and the particles inside it start moving around a lot. Just like how your hard ice cream gets all drippy and runny when you leave it out in the sun for too long!
Okay, I hope that all makes sense! Let me know if you have any more questions or if there's anything else you want to learn about!