Science Buddies: "Ask an Expert"

I am testing toothpaste,gel and baking soda on egg shells to observe how white they get or which product is better to whiten your teeth with.
The original plan was to dip the egg in coffee, then apply the product, record the results after every hour and then repeat everyday.
Then my partner suggested we do not dip the egg in coffee and just use the egg shells as they are.
I have also wondered if we dip the egg in coffee once and then apply the product everyday without re-dipping it in coffee will work the same.
My partner's mother works as a dentist assistant and she said that just using the egg shells as they are is fine.
I am having doubts about the way to do things, so I asked my science teacher and he said he doesn't know (which is not very helpful).
Can you help me pick which method I should proceed with?
Thank you

Hi Jpham,

The most important thing is to pick one way and always do your experiments that exact same way. I think any of the options you listed below could give you interesting results, so you can just pick one and run with it. Keep in mind that each of these experiments will answer slightly different questions. For example, dipping the shell in coffee every day would assess how effective the whitening products are at removing stains as they build up. Dipping the shell in coffee once would assess how effective the whitening products are at removing stains that are already present on the teeth.