For a particle you create with the Particle Tool, a creation expression executes when you rewind the animation. For an emitted particle, a creation expression executes in the frame where the particle is emitted. However, there are exceptions to these rules as described in the following topics.

Rewinding an animation two or more times in succession without playing the animation doesn’t execute a creation expression. Because no attribute value changes when you rewind several times in succession, the expression doesn’t execute.

You might also notice that all expressions in your scene are compiled and executed each time you open the scene. This occurs for architectural reasons and is unimportant to your work with expressions.