  Several abundance analyses of Galactic open clusters (OCs) have shown a
tendency for Ba but not for other heavy elements (La$-$Sm) to increase sharply
with decreasing age such that Ba was claimed to reach [Ba/Fe] $\simeq +0.6$ in
the youngest clusters (ages $<$ 100 Myr) rising from [Ba/Fe]$=0.00$ dex in
solar-age clusters. Within the formulation of the $s$-process, the difficulty
to replicate higher Ba abundance and normal La$-$Sm abundances in young
clusters is known as {\it the barium puzzle}. Here, we investigate the barium
puzzle using extremely high-resolution and high signal-to-noise spectra of 24
solar twins and measured the heavy elements Ba, La, Ce, Nd and Sm with a
precision of 0.03 dex. We demonstrate that the enhanced Ba {\scs II} relative
to La$-$Sm seen among solar twins, stellar associations and OCs at young ages
($<$100 Myr) is unrelated to aspects of stellar nucleosynthesis but has
resulted from overestimation of Ba by standard methods of LTE abundance
analysis in which the microturbulence derived from the Fe lines formed deep in
the photosphere is insufficient to represent the true line broadening imposed
on Ba {\scs II} lines by the upper photospheric layers from where the Ba {\scs
II} lines emerge. As the young stars have relatively active photospheres, Ba
overabundances most likely result from the adoption of too low a value of
microturbulence in the spectum synthesis of the strong Ba {\scs II} lines but
the change of microturbulence in the upper photosphere has only a minor affect
on La$-$Sm abundances measured from the weak lines.
