here are 3 version of the same gel. the image shows the degradation of the active ingredients (antioxidant) over time. as it gets dark, it loses activity.
<a href="//commons.wikimedia.org/w/index.php?title=User:Lunadei34&amp;action=edit&amp;redlink=1" class="new" title="User:Lunadei34 (page does not exist)">Lunadei34</a>
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<span class="int-own-work" lang="en">Own work</span>