how do particles know when they are being observed?
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OP, every other answer in this thread is wrong at the time of this comment, because they erroneously imply that the particle is perturbed by the act of measuring, which changes the value from what it was previously. Quantum uncertainty is a fundamental physical limit on the accuracy with which a quantity can be know, and even with the best non-intrusive measurement equipment there would still be this uncertainty.
You’ve probably heard before that particles have wave-like properties. In crude summary, what this means is that at the quantum level, the location of a particle is defined by a spread of probabilities called a wavefunction. It is not in one place, waiting for us to detect it at a specific location within this probabilistic range. It has no 100% precise fixed location with hard boundaries. Because it is a wave, not a particle.
When an interaction occurs (physical, chemical, etc.), a particle is forced to pick a specific state in order for the outcome to be calculated. This is called observation, and does not need to necessarily be conscious. These interactions are occurring away from human sight constantly, where particles defined by probability are briefly forced to “fall into” a fixed state by the world around them. This is called a collapse in the wave function, and is usually what people refer to when they talk about “particles behaving differently when observed”.
I know this is overly long for an ELI5. But TL;DR: The particle is not being “tapped” or “knocked” or “shifted” such that its state is changed. Rather it is being forced to “fall into” a fixed value amongst many superimposed probabilities, in order to participate in an external interaction.
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