It's moooore "transmyooooting into" than "storing", really. The udder full of moooolk situation is just a not-so-surprising effect of being moooode of moooolk. To answer the gist of the question, almoooost all kinds of liquid, sqrrk! Som-oooodeas without going in depth about them:
- Fruit juice
- Energy drink
- Moooolten salt
- Slime (like the RPG moooonsters)
- Liquid latex
- Something flammooooble?
- Oat moooolk