You're teaching an exercise class. You're leading the class in stretching. You want to tell them how to breathe.
Inhale... and exhale.
breathing, exercise, instructions, science 
Your kitchen stove doesn't work. You call your landlord to fix it. You explain the problem to him:
You can hear the gas coming out when you turn the knob, but it doesn't ignite.
action verb, home, repairs, science 
You use fluorescent light bulbs in your house to save electricity. You're explaining why you use them to an acquaintance who says he doesn't know why people use them. You say:
They're more energy efficient than conventional bulbs because they generate a lot less heat.
comparison, electricity, energy, explanation, science, technical You work at a new research laboratory at a university. You're giving a tour of the lab to a group of visitors. You say:
This is our recently built state-of-the-art laboratory facility.
adverb, formal, laboratory, science, technology, tour, university 
You're reading a website about penguins. It's written in a kind of scientific style. One part of the page is about how they swim. It says:
Their powerful flippers propel them through the water in search of prey.
animals, hunting, movement, science, written English 
You're reading a web page about penguins written by someone who's really interested in them. The page is explaining penguins scientifically. It says:
Penguins are remarkably well adapted to their icy environment, thanks to millions of years of evolution.
animals, environment, science, written English 
It's a cold day outside. When you come inside your warm house, the glasses that you're wearing get moisture on them and it's hard for you to see through them. You say to your sister, who's sitting in the house:
My glasses are fogging up.
cold, glasses, progressive aspect, science, weather 
You're talking to a child. You're explaining why the temperature is different at different times of year. You say:
As the Earth orbits around the sun, the angle at which sunlight hits the Earth changes.
Use this kind of grammatical structure to describe two things that happen at the same time. It's most commonly used for two continuing processes that happen together:
As you get older, you learn what your own strengths and limitations are.
"As ___, ___" is good for describing how things work. Here's a technical description of how water boils:
As the temperature of the water rises, some of the water molecules begin to go into a gaseous state.
To "orbit" something means to travel in a circular path around it. Planets orbit the sun, and the moon orbits the Earth.
Technically, the word "around" is not needed. A more grammatically correct sentence would be:
As the Earth orbits the sun, the angle at which sunlight hits the Earth changes.
But other phrases use "around" to describe the same motion, like:
So adding "around" to "orbit" might make it easier for someone to understand if they're not familiar with the meaning of the word "orbit".
The example at top includes the phrase "the angle at which sunlight hits the Earth". This is a reordered version of "hit (something) at an angle":
The light from the sun hits different parts of the Earth at different angles.
When you reorder a sentence, you still have to keep the preposition "at". There are two choices for where to put it:
As the Earth orbits around the sun, the angle which sunlight hits the Earth at changes.
As the Earth orbits around the sun, the angle at which sunlight hits the Earth changes.
The first version is a lot more casual. The second version sounds more intelligent. It's also considered more grammatically correct, so please use it when you're writing or answering a grammar question on a test.
This same structure works with other prepositions:
children (talking to), climate, explanation, formal, science, weather 
You're talking to a child and explaining why there are different seasons. You start out with
The Earth's axis is tilted.
Earth, adjective, children (talking to), explanation, math, science, seasons, turning 
