40d:Blood

The favoured drink of Armok, Blood (or ichor, or your local bodily fluid's name) is found in most of the creatures in Dwarf Fortress, with the notable exception of skeletal creatures (nothing to hold blood, so why should a bone bleed?). It comes in several colours, red for most animals and creatures, and eg aqua/teal for Trolls. This stuff usually ends up covering anybody involved in melee combat (especially dogs, cats and wrestlers who must physically be on their target to dispatch them), and will inflate creature inventory screens very quickly. It is not known if you can make a 'dwarven washing machine' (essentially a small, drainable water-trap with a cage for the animals and a glass wall for people to see inward) to clean them up.

Bleedin' Bugs

A known issue with blood, especially that which spatters (or drips, or in elephant heart shots, gushes out in massive torrents), is that trudging around tiles which have blood on them will spread the stuff to nearby tiles, which will reset the 'drying-up' counter on both bits of blood. This leads to your fortress quickly being tinted a nice red colour, due to blood which spreads much like Tiberium in the C&C universe. This is no problem if your objective was to dedicate your fortress to Armok, but if you like the natural colours of your local rocks/sand/clay, it can be a problem. One solution to this is to designate a d-o low or restricted traffic area over the problem area, and possibly tunnel a back route around should the blood be indoors. This will stop dwarves going over it (hopefully), which will allow most of the blood to dry out.