Some caution is required in describing structures on a cosmic scale because they are often different from how they appear. Gravitational lensing can make an image appear to originate in a different direction from its real source, when foreground objects curve surrounding spacetime (as predicted by general relativity) and deflect passing light rays. Rather usefully, strong gravitational lensing can sometimes magnify distant galaxies, making them easier to detect. Weak lensing by the intervening universe in general also subtly changes the observed large-scale structure.