def _perturb_ftr(ftr):
    """Displace a single particle. 
    
    Return the ID of the particle, and the perturbed DataFrame."""
    ftr = ftr.copy()
    pid = ftr.particle[(ftr.x == 15) & (ftr.y == 15)].values[0]
    ftr.loc[ftr.particle == pid, 'y'] += 0.1
    return pid, ftr