Why the key in the challah? So many people misunderstand this custom and therefore think that it is foolishness. The fact is, as always, the custom makes a lot of sense when we understand its meaning.
We are told that on the first Shabbos after Pesach placing a key in, or on the challah, or forming the challah into the shape of a key is a segula for parnassa, a remedy for our livelihood.
The custom is to remind us the reason we have two challahs (loaves of bread) on Shabbos. It is to remember the Manna in the wilderness.
When we went out of Egyptian slavery, out into the wilderness there was no visible sustenance. Then Hashem provided for us miraculously by providing the Manna lying on the ground right in front of us. But one of the biggest lessons of Egyptian slavery was/is that if we take more than we need the extra will rot. So how then could we provide for the family's needs on Shabbos when we were not allowed to go out and pick up the manna? The manna that we picked on Friday would miraculously double and provide us with our needs for Shabbos. This is the reason for the two loaves on Shabbos all year long, to remind us of the double portion that Hashem miraculously provided for us in the desert.
The key to our successful livelihood is in remembering to go out into the world to make a living as the tribe of Levi did in Egypt. They went there with Torah and Mitzvahs and not with the greed that comes from just focusing on money. This is an ongoing lesson. If we go out into the world to make money and the money is our sole purpose we are going to become enslaved just as we were in Egypt. But if we go out into the world to that same job, to earn that same money in order to use that money for a life of Torah and mitzvahs, that same work will become holy work.
This is why remembering the reason we have two challahs on Shabbos is the key to a successful livelihood.
Have a nice Shabbos.