As any good scientist does, we hypothesized and made predictions about the task at hand: What would happen if we put a Peep in a beaker of 200 mL of water? What will happen if we put a Peep in 200 mL of vinegar?
Some ideas: Sugar dissolves, so the Peep will dissolve. Vinegar is acidic, so it will dissolve the Peep faster.
After letting the Peeps sit for an hour, we discovered only the sugar on the Peeps in water dissolved, but the marshmallow portion was still in tact. As for the vinegar, it was bubbling a little bit, but the sugar was dissolving at a much slower rate than the Peeps in water. Again, the marshmallow was still in tact!
Click the link to see a similar experiment, and possible explanation for why the Peeps didn't completely dissolve: http://www.peepresearch.org/solubility.html