On the Menu: Chocolate Chip Cookies
Recommended Reading: I'm Just Here for More Food by Alton Brown pgs. 14,15;86-89;90,91. [For less advanced readers, read it together.]
Rader's Chemy4Kids.com online lesson on Solutions & Mixtures. [For children with no exposure to chemistry basics, do the entire lesson on Matter up to and including Solutions & Mixtures.]
Recipe: I'm Just Here for More Food by Alton Brown pg. 96 Chocolate Chip Cookie #10.
Question: Does weighing ingredients by weight or by volume make a difference in the outcome? Why or why not?
Inexperienced Educators Concern: But I don't already know the answer to the question. I don't know what will happen. All the experiments in the book have a "right" way to do the experiment and a "right" answer.
Wise Cook-in-Residence: Real scientists don't know the answer. Their hypothesis might be more educated their ours but they don't know for sure. If they knew the answer already, why would they need to experiment?
Hypothesis1: Measuring ingredients by volume or by weight will not make any difference in the outcome of the cookie or how it tastes.
Hypothesis 2: Measuring ingredients by volume will make better cookies because that's how most recipes are written.
Procedures: We made two batches of cookies. One was measured by Volume and one was measured by Weight. This includes all dry and wet ingredients, including egg yolks and melted butter.
Observations: 
Measuring by Volume is certainly easier and faster. Measuring by weight, it would be best to have a scale with a tare feature (you "tear" away the weight of the container sitting on the scale).
It was also observed that 2 sticks of butter by volume (2 cups) equals only 7.7 ounces not the 8 ounces it should. The volume of 2 egg yolks was also found to be a bit "open for interpretation" because not all eggs have the same size yolk (weight measurement was simply 2 large egg yolks).
The dry ingredients from both batches appeared to be identical. (In the future, planning ahead, we would have put the dry ingredients in identical size bowls and also have weighed the empty bowl so we could weigh the dry incredients of both batches.)
The wet ingredients did show a visual difference. The batch measured by volume was lighter in color than the batch measured by weight. The initial guess as to the cause was either a differing amount of vanilla or a differing amount of brown sugar.
Final Procedures: Both batches were cooked at the same temperature, in the same oven, for the same length of time. Each batch had a "turn" on the top rack or the bottom rack of the oven - so that half of each batch would be cooked in the same position.
Cooked Observations: The cookies' color difference remained after cooking: one was noticably darker than the other. In addition, the darker batch appeared to be wider and flatter. Measuring proved this to be true: Batch by volume was 7 cm wide (on average). Batch by weight was 7 1/2 cm wide (on average). Six cookies from each batch were also weighed. The average weight of the cookies from each batch was equal = about 1.1 ounces.
Initial Hypothesis Analysis: In this instance of the experiment, the measuring of ingredients by weight or by volume appeared to make a difference in color and size of the cookie. However, both hypotheses were stated as an analysis of the taste of the cookie.
Why are the cookies different in color? Upon discussion with cook-in-residence, Daddy, all scientists agree it wsa the brown sugar. We remembered that during the Volume measurement there was some question as to whether or not the brown sugar should be packed down tight or left loose. Because the recipe did not specify "packed brown sugar", the ingredient was measured loose. Obviously, they concluded, this resulted in less brown sugar in the volume-measured batch. In addition, measuring by weight showed to be more accurate because it would not matter if you packed the brown sugar or not (if you sifted the flour or not), it would always weigh the same. [This notion is supported in the recommended reading in Alton Brown's cookbook.]
Why are the cookies different in shape? Brown sugar is made of crystals. When it is cooked it melts to be kinda syrupy. Syrup spreads, both scientists described. So, if the by-weight cookies had more brown sugar, there would be more syrup to spread and widen the cookies.
Secondary Hypothesis Analysis: Both hypotheses were phrased stating that the different techniques in measuring would affect the taste of the cookies (or in one case would NOT affect the taste of the cookies). After a brief discussion of how you measure taste, both scientists were certain that taste was a conclusive measurement. Cook Daddy decided to admister a blind taste test. Each scientist was given 4 bites of the cookies (2 of each batch type) with their eyes closed. Neither scientist was able to accurately identify which 2 bites (1st, 2nd, 3rd, 4th) were from the same cookie. However, both scientists did identify 1 bite from "their" batch as their favorite tasting cookie.
Taste Results: Inconclusive. Taste is not a useful or measurable scientific description. Perhaps if the Hypothesis had stated there would be differences in appearance or differences in weight, it would have been more relevant.
Final Hypothesis Conclusion: Because the hypothesis was based on taste, an unmeasurable attribute, we have no formal conclusion.
Inexperienced Educators Conclusion: But we didn't answer the question or any question? We don't know for sure why they are different. What did I do wrong?
Wise Cook-in-Residence: In science, you rarely answer your question or proove your hypothesis. Usually you just end up with more questions.