Cecum

It's like the murine equivalent of an appendix, only with a digestive function and not just as a reservoir for normal gut flora. The jejunum feeds it and the ileum drains it, but it's pretty much a big J-shaped sack that holds digesting food. I do not know what it's contents would look like in a wild mouse with a heterogeneous diet, but in lab animals with a homogeneous diet it is uniformly brown (think pre-poop).

I bring up the cecum because I dissected out possibly the biggest cecum I have ever seen the other day. It was from a Germ-free Swiss Webster mouse, about 18 months of age, who was suffering bloat from a downstream gut twist. Ceca tend to bloat anyway in germ-free mice, and the gut twist may have backed up materials as well (although it pooped on me anyway when I picked it up), but the mouse still had retained ~3g of cecal contents (mouse weighed maybe 25g). That's incredible!

If I had the same relative mass of undigested food in me, that'd be about 9kg!

(I have a morbid tendency to calculate things upward from mice to my own mass. For example, the LD50 of Stx-2 (Shiga-like-toxin-2, produced in very virulent O157:H7 E coli strains) in mice is about 1.3E-5g/kg [if I am remembering the paper on it correctly]. Which means that the LD50 for me would be only about 3mg!).