microscope
Magnifying glasses
Telescope
Digestion.
Small bacause if you have large you could hear things you are not supposed to
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A general answer to your question would be: tools. For example, a microscope is a tool that makes it easier to understand things that are too small to observe directly. An x-ray machine is a tool that allows us to see inside of things that are too hard to see inside of directly. A knife is a tool that allows us to cut up large things to see what is inside of them.
An atom is very small the human eye can not see an atom that is what makes it small.
microscope
piano
Microscopes
Microscope. Or a magnifying glass