If two things interpenetrate, each spreads through the other. They do not stay in neat separate layers. Light may interpenetrate a thin cloth. Two cultures may interpenetrate in a city until you cannot say where one stops. Ideas in a book may interpenetrate until you cannot pull them apart. One thing can also interpenetrate another by spreading all through it. It is a formal, rather academic verb. Scientists, philosophers, and careful writers use it. Ordinary speech almost always says “mix”, “spread through”, or “go through each other” instead. You do not need this word to buy fruit or tell a friend your news. The shape is honest: inter- (“between / each other”) + penetrate (“go into / go through”). So it is penetrate, but both ways, or all through. Though actually, penetrate can sound physical and sharp; interpenetrate is often used more softly, for ideas, cultures, light, and space, not only for one object pushing into another.

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