XD Here's a hint, the title's initials of the work that I got the costume from is BL.

Uh...XD; well, the idea (if it really is milk, which at this point I just think it's pointless to argue...XD;) is that HK spat out the milk on UK and then UK's all "ARGGH You little brat XDDDD" and playfully threw the whole cup over him.

...or not.