Light UpTo light up can mean to add a small flame to something, for example a gas stove or a cigarette. It can also mean to add enough light to something that it can be seen clearly, as in a lamp that lights up a room.
Ex: James went out of the bar for a few minutes to light up. He'd been wanting a cigarette for a while but you cannot smoke inside the place anymore.
Ex: I didn't realise how ugly the wallpaper was until I turned on the lamp. Once it lit up the room, I could see how terrible the peach wallpaper looked with the blue furniture.
Light up can also be used figuratively to describe when a person becomes happy and their happiness is visible on their face, because they smile and their eyes look happy. You can make it more specific by saying a person's eyes light up (with joy).
Ex: When the conversation turned to her children, Maria really lit up. She's obviously so proud of them, she loves to talk about them.
Ex: The best part of Christmas morning is when the kids first see the presents under the tree and their eyes just light up with joy. It's impossible not to feel touched by their enthusiasm.
SimmerTo simmer is to keep a liquid on steady heat, usually without boiling. Many recipes require you to bring a pot of liquid to a boil and then reduce it to a simmer for some time.
Ex: The recipe says that you shouldn't boil the sauce, but just put it on a medium heat and simmer. I think boiling milk will make it separate.
Ex: Sally boiled the water, added some rice, and then reduced the heat so that it could simmer for a while.
When someone is angry or excited, they may need to simmer down, or become more calm. Parents often tell very active children to simmer down. When a situation or conflict begins to get calmer and more quiet, it has also simmered down.
Ex: Nancy has been running around yelling at her employees all morning. She really needs to take a break and simmer down. Being like that isn't good for anybody.
Ex: John and Julia had been arguing for quite some time before I suggested we take a walk. I waited for them to simmer down a bit and then proposed a stroll.