Mirrors in the Temple.
Vayakhel-Pekudei (Exodus 35-40 )
We can't enter God's presence without first taking a fearless moral inventory of ourselves.
As we conclude the Book of Exodus, we are given a list of all the gifts the Jewish nation gave to build the Temple: "gold, silver ... precious gems." The midrash points out that of all the donations made to the Temple, the mirrors that were used for the sink were the most precious to the Almighty. Why was such a valued gift used for a sink?! It seems strangely out of place.
One of the purposes of the mirrors was to remind us that we can't enter God's presence without first taking a "fearless moral inventory of ourselves," while at the same time acknowledging all the blessings we have been given gratis. We might be able to blame everyone around us for our mistakes, but before we face God, we need to look in the mirror, acknowledge our shortcomings, and "humbly ask him to remove them." We then have to wash away all our excuses and rationalizations, all our pettiness, and take the opportunity to use our blessings to do the right thing throughout life.
That's one of the reasons the mirrors were so precious; they forced people not to be satisfied with the status quo, but to grow and be big. It's no coincidence that the first mitzvah a Jew performs every morning is washing his hands before he starts his day. Before we enter God's world (the Temple), we need to look at ourselves, wash away the nonsense and pettiness that hold us back, and make the most of the opportunities the Almighty places in front of us.