I am a father of a two-year-old girl, and as she speaks more and more from day to day, I am already impressed by some questions, like "why does it rain?". Of course, she does not ask me this with any physical ulterior motives, but maybe in some years.
As I read about teenagers, this "interest" fades away. What is sad, because IMO asking questions, being interested in things, is the fundamental of learning.
My question
How can we keep up this (seemingly) natural interest in our environment?
Just to complete Xander's answer, two very important factors imho:
1. Answer her questions !
Sometimes you may have other things to do, or it may seems too hard to explain why this or that in a way a toddler can understand ("you will understand when you're older" is one of the worst things to tell a child), but it is important to always show her that her curiosity is not vain. If need be, tell her that you'll explain later if you have no time, or tell her a simplified version and precise that there's more for her to discover later on.
Don't forget there are lot's of different good answers to any question depending on her age but also on what topic you'd like to bring into the conversation. For instance "why does it rain ?" can be "because the water in the cloud is attracted by the Earth" if you want to talk about physics, "because we leave in an area where it rains a lot in August" if you want to talk about geography, or "because of the cycle of water" if you're more in the mood for natural science.
2. Give her the right example
Children often believe that adults, especially their parents, know everything, and don't need to learn anymore. It may make them feel that learning is a secondary activity, for kids only, not valued among adults. If your child sees you reading a book about astronomy, or learning Spanish, or studying for professionnal aims, it will encourage her in her learning process.
Also, don't be afraid to tell her sometimes that you don't know something if that's the case; but it should be "I don't know, I'll look about it in a book / on the internet" rather than "I don't know, don't bother me with this".
(just the personal pow of a father of two curious children)
How can we keep up this (seemingly) natural interest in our environment?
Model the behavior.
Right now you have most of the answers, which is great, but not so great when teens start to reject your ideas/authority figure status. So, make it a habit.
At first, just giving answers is fine, but it's even better to show kids, and it's not too early to start if the child is close to three. Experiments are fun, and simple things link baking soda and vinegar being able to blow up a balloon or be able to produce an invisible gas that can put out a candle are like a magic act to a kid! Just keep them short when very young and age appropriate. A prism or crystal to split light into rainbows is wonderful. Awesome things surround us!
Clouds are water vapor in the air. Heat water (not to boiling quite) and hold a big spoon over it that you just took out of the refrigerator. She can't yet see the water moving through the air, but there it is on the spoon. Just like in the clouds.
Say, "I don't know", and look things up together.
Propose questions of your own that might interest her. What will fall faster, a big marble or a little one? Why?
Rent books from the library on science and the environment.
My kids grew up in a two-physician home, so science and the scientific method was always a topic of conversation and a way to prove our 'beliefs'. I was also a naturalist by avocation and a molecular biologist by training, so I gave my kids diverse books that I thought they'd like. It is a big part of their lives still. (My kids know wildflowers, birds, trees, etc. My oldest was into survival on Nature alone, so just loved books like Hatchet and the Tom Brown survival guides. Cater to their preferences.)
Don't overwhelm them.
Do lots of other fun stuff, too. Balance it with other things they might like, like art, sculpting (who doesn't like to play with clay/Sculpy?), etc.
Have fun! It's not fun for them if it's not fun for you, and vs versa.
One of my favorite stories about teaching my kids: I was picking up after homeschool around noon, and I had intended to teach about the hemispheres but didn't get around to it. My oldest wandered in with a question he had about something unrelated (I think it was a TV show) and at the end of the conversation, I found myself with an orange and two rubber bands in my hands. "You know, 'Sam', if this orange was the earth..." "Mom!" interrupted my son, "I didn't come in here to learn anything!" I had to laugh and admit defeat.
Remember that you can only steer her towards science, you can't know how her interests will develop till her teens.
Until she's ready to make choices, you could firstly help her learn by example. Teach her about recycling, conserving water and another ways to protect the environment.
Secondly you can help her by providing her with related children's books. There's a plethora of kids books about science. Read it to her or with her and explain any concepts that she finds hard or interesting. Personally since both my wife and I are psychiatrists, we have tried to steer our children towards that by giving them books (they're 4 and 5) and also discuss topics related to psychology with them. They were both fascinated by mum and dad's mind powers.
Lastly, you could also look into toys related to science, such as science kits.
Remember that the best way to engage children with any topic is to help them think for themselves. Making them think or do research on a topic is always more beneficial than providing the answer yourself. Helping in their quest is also a great way to bond.
I applaud you for wanting your child to learn more about the environment. It's most likely that our children will face the greatest difficulty when it comes to Earth's environment and being conscious about it is important.
Ps. Although I am a psychiatrist, I am not that well versed in children psychology so these opinions are those of a father rather than that of a professional.
Make sure it works.
People are expected to keep doing anything that works. If her asking questions is meeting her needs she will keep at it. Taking the time to teach her something about rain is good, she likes the attention and likes that her world becomes a little more clear.
One of the main reasons teens often show less interest in curiosity is social. They may receive negative feedback from peers or once they match the understanding of the adults around them. This isn't expected to apply to children since their peers are also awed by everyday wonders, and the adults all still have better understandings.
To avoid this fading over time either isolate them from peers and adults who have covered or lost their curiosity (home school seems to work reasonably well at this in my indirect experience) or nurture a hobby where it will always be rewarded (even beanie babies offer fuel for curiosity) and see to it she has like minded acquaintances; when she passes your ability to teach about it introduce her to a local club or an internet community.
Already upvoted answers are great. I'd add that when you don't know the answer, tell your child "let's find out" and take your child with you to books or internet and look for the answer (images and videos are better than reading text ofcourse). That will get them motivated, they'll know they can find out anything and it might encourage them to ask for more questions.
I have a slightly different take on this than the other answers.
I answer questions with stories rather than facts.
Why does it rain?
Let me tell you about the terrible morning the cloud passing over us had.
Let me tell you about the Sun is doing his laundry.
Let me tell you how water learns to fly...
One of these answers is close to the truth. .. This works because it gamifies the act of asking questions. The asker is ensured to get a good story out of this and is incentivized to ask more.
Critique of other proposed approaches - Giving them the answer - scratches their itch right now but doesn't incentivize increase questioning - Showing them how to find the answer - teaches self reliance but doesn't make asking questions more enjoyable.
We do end up reading books for answers after the silly stories have piqued her curiosity and she wants to know which one is real.
