Crabbe and Goyle guffawed, but before they could go any further Hagrid emerged from the back of his cabin balancing a teetering tower of crates, each containing a very large Blast-Ended Skrewt. To the class's horror, Hagrid proceeded to explain that the reason the skrewts had been killing one another was an excess of pent-up energy, and that the solution would be for each student to fix a leash on a skrewt and take it for a short walk.