Whenever Xander had to give Giles bad news, he had a habit of measuring how long it took Giles to clean his glasses.
Whenever Xander had to give Giles bad news that was sort of maybe a little bit his fault, he always found himself hanging out for the glasses-cleaning. Giles with glasses on was scary teacher guy who could probably give Xander a detention, or use some of his Watcher powers to turn him into something. Without his glasses, he wasn't nearly so worrying.
Thus, Xander was trying to draw out this particular bad news as long as possible, so that Giles would hopefully take off and clean his glasses before Xander reached the /point/.
"OK, so, you remember how when Buff was taking out that last vampire nest, she found this vat of yellowy fizzing stuff? And how the vamps were going to feed it to those guys they'd captured before they ate 'em? So, well, she filled a Thermos with the stuff and brought it back here, and me and Will were looking at your books about it, to find out if they were, you know, trying to summon Astaroth to pass higher level history, which, let's face it, we've all thought about at some point or other, and then Oz came in, and he took the top off and sniffed it a little, and he said, 'It's just Mountain Dew'..."
"Some would call that more than demonic enough," Giles interjected.
"Well, yeah, what with the snowboarders and all. So, anyway, I started laughing, and... I started laughing /really hard/, 'cause I'd been working on that for a while..."
"Library regulations do say that one should remain quiet, yes, but I hardly think it's something you really need to confess to, Xander."
"Well, when I was laughing, I kinda moved my hand, and I kinda knocked over the Thermos, and I kinda... spilled Mountain Dew all over your copy of the Names of the Lords of Hell."
At /that/, Giles took off his glasses, and rubbed them with the handkerchief he kept in his pocket for just such a purpose, and Xander was relieved.
/Forty seconds/, he said to himself.