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## Visualize internal motion using marker particles

Figure 1. Marker atoms in a deforming crystal

Marker or tracer particles are a way to make the internal deformation or diffusion within a system visible as shown in the simple example on the right. The following recipe explains how to make sure that the set of selected marker particles, once defined on the first frame, remains the same over the entire course of the simulation.

The example animation has been created as follows:

1. First we apply the Slice modifier at frame 0 of the simulation (the initial state) to select a stripe of atoms. Activate the modifier's Create selection option to let the modifier select the atoms instead of deleting them.

2. We then apply the Freeze property modifier and choose the `Selection` particle property as the one to be frozen. The modifier automatically takes a snapshot of the particle selection state generated by the Slice modifier and preserves it over time.

3. Finally the Assign color modifier is used to assign a special marker color to the selected particles.

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