In this kind of building set, there are a limited number of types of block and each block has a particular shape. Just as importantly, each one has a particular way in which it can link to other blocks because of the way the studs are arranged.
The blocks can help you see how the atoms link in a molecule of water. Look at Figure 7 where the red brick represents an oxygen atom and the white bricks represent hydrogen atoms. There are only two locations where the hydrogen atoms can join the oxygen atom - at the top and bottom - as shown in Figure 7.
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