Matzah. If you aren't Jewish, imagine a cracker with no shortening, no butter, no nada. Just flour and water, mix and bake. It's a big part of the Passover diet, because breads, pastas, cakes, etc. are all off-limits. Whazzup with that? Aish.com explains:
"Why is matzah so basic to the celebration of Passover? Why is Passover called Chag HaMatzos, "the Holiday of Matzos" by the Torah? Why is this simple food a foundation of Jewish experience and ideology? Why has matzah come to symbolize human freedom?"
It's all about fermentation, deliverance - and time. An interesting read.
Another leavening issue for the Israelites leaving so quickly is that without our modern packaged cultures, they had to rely on either the ambient contamination of the mixing vessels or small samples of the prior mix that had risen and not been baked, but rather reserved for leavening the subsequent mixes. In the process of such things, in an age without concept of sepsis or antiseptics, the bakeries started to smell like breweries (due to the same yeasts being used, unintentionally) and occasionally like cesspools when the retained samples (called "mothers" in baking today) went really sour.
The image I get of the matzos instruction was that the Israelites would leave uncontaminated AND not bring the contaminated "mothers" of Egypt with them, as this proscription would have forbidden bringing the souring cultures from Egypt as well. A clean break with the past, but that leads directly to the manna story as well, because in the desert, it probably was extremely difficult to get bread to rise reliably being on the move and being shorn of their "mothers".
These days it's not matzah that's such a large part of the Passover diet but products made from matzah meal (ground up, virtually powdered matzah). Many of these are very adequate replacements for the proscribed bread, cakes, etc. eg. desserts like chocolate mousse matzah layer cake, matzah meal brownie cake.