  Although there is no consensus regarding the "reality" of the past of a
quantum particle, in situations where there is only one trajectory with
nonvanishing quantum wave of the particle between its emission and detection
points, it seems "safe" to associate the past of the particle with this
trajectory. A method for analyzing the past of a quantum particle according to
the weak trace it leaves is proposed. Such a trace can be observed via
measurements performed on an ensemble of identically pre- and post-selected
particles. Examples, in which this method contradicts the above common sense
description of the past of the particle are presented. It is argued that it is
possible to describe the past of a quantum particle, but the naive approach has
to be replaced by both forward and backward evolving quantum states.
