While teaching STEM this year I always start with a little bit of research for background information. There are many ways to do this such as reading a picture book or an article, manipulating objects such as toy boats or zip lock bags, even watching a short video. The CK-12 Foundation has brought a wide-range of interactive simulations to explain different science topics.
CK-12 Foundation, is a nonprofit that aspires to increase worldwide K-12 access to “high-quality curated STEM content”. CK-12’s main offering is a collection of free digital textbooks, “Flexbooks”, for high school age students. Along with the Flexbooks are videos, quizzes, flashcards, and these spectacular “simulations”.
Each simulation starts with an introductory animation to provide an explanation of the topic at hand. After, students interact with the concept by changing variables to see different effects and expand their knowledge on the topic. Sometimes this is followed by a “Challenge Me” section in which students answer questions about the topic.
The first two sections are great for introducing any science topic or STEM lesson.
The user can filter content by science concepts. Each topic has sub-topics within it.
As you scroll you can see a lot more “coming soon”. It is great to know more simulations are on their way. 
There is a large amount of content covered for older students, high school. When looking for younger students I found it to be a bit complicated without detailed teacher explanations and assistance.
For example the see-saw, a simple machine and a topic taught in second grade at the school I work in, became complicated during the interactive part. The animation was helpful, the simulation, above their grade level.
There are some simulations like The Runaway Balloon which is slightly more age appropriate but still needs teacher explanation.
