For the most part, they are water proof to a point, but they aren't completely. The easiest way to do it is to let the fuse burn down a little bit and drop it in when it is close to the edge of the firecracker. Once it gets in, hopefully as it is dropping, there is a small amount of room for it to burn within the fire cracker itself, and water won't have a chance to get into it. The fuses are supposed to be water proof though, it just doesn't always work.