When Krillin and Gohan are going back to Kame House while Goku is fighting Vegeta. Gohan stops and looks at the artificial moon that Vegeta created. Krillin says that it is impossible for the moon to be there because Piccolo destroyed the earth's moon. How does Krillin know that?
Yeah, that's pretty stupid (Gohan couldn't have told him because he was Oozaru when Piccolo blew it up). But, the biggest slipup is actually the fact that the moon was already destroyed once! In Dragonball, Kamesennin (Master Roshi) blew up the moon with a Kamehameha because Goku went Oozaru. The explanation? The episode where Gohan became Oozaru was filler, and the guys making the fillers don't care about history or Power Levels or things that should be impossible (like Goku using Kaioken WHILE IN SUPER SAIYA-JIN at the Anoyochi Budokai).
There could be several explanitions for this, have you also noticed in one of the epsoides where Frizza and King Cold are going to earth they show there ship siloetted by the moon? well anyway maby people kept wishing the moon back? Since it isn't a liveing being so it can be wished back several times. also has anyone noticed in a eposide in the Cell Saga, Goku uses the Ka-Ma-Ha-Ma-Ha and it disapperas in space? yet when Piccolo was way weaked and used a weaker blast his reached the moon? Also in one of the movies(Worlds Strongest) they fight for a bit in space but...they do axpyesate. oh well guess that means its just a cartoon
The question isn't how does kuririn know, it's how did piccoro blast the moon. If you remember from "dragonball" in the first "Tenki ichi budoukai"(sorry if misspelt, but it means world martial art tournament in FUNi version) when Goku faces off with Jakie Chun(Muten Roshi), Jakie blasts the moon so goku isn't an oozaru(monkey) any more, and alot of people get mad at Jakie for it. Oh yeah, one more thing, Goku means monkey(pun because he turns into an oozaru and kills his grandpa that way.)