Green ants!

Exactly what you want crawling around your beer

To Liu Nineteen 
Bai Juyi

Green ants! On fresh-brewed ale there's busy foam,
Red clay! My parlour's terracotta stove,
It's late, that sky looks like it's going to snow,
Come drink! My friend, the nineteenth son of Liu!

白居易  问刘十九
绿蚁新酿酒，红泥小火炉。
晚来天欲雪，能饮一杯无？

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In old China, children were sometimes literally named One, Two, Three… after their birth order. Even when that did not happen, it was still common to use birth-order numbers to refer to or address friends. This is why Bai calls his friend, whose name was Liu Yutong, Liu Nineteen. In a large, aristocratic family, birth order might be counted across an entire generation, including not just the siblings in one nuclear family, but all first cousins (and maybe even more distant cousins). So Liu Yutong was the 19^th child (possibly the 19^th boy) born in his generation of the large Liu clan.

When rice wine has finished fermenting, it is greenish in colour, and little bubbles scurry across its surface; these bubbles were called "green ants." This picturesque piece of brewer's terminology formed the basis for Bai Juyi's poem. It's a very slight piece, so much so that I was tempted to give it this alternative translation:

Hot coals are red,
New wine is green,
It's going to snow,
Come drink, Liu Nineteen!