Addressing the “4-year-old wants to know” part of your question rather than the thread title, it’s worth noting that plenty of substances are transparent to other parts of the EM spectrum. Radio waves can pass through wood and drywall, while light can’t even make it through the paint. X-rays can go right through your body, but UV is blocked by glass or [sunscreen](https://youtu.be/GRD-xvlhGMc).
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