True story: I grabbed my camera, went to the beach, and just when my daughter was doing something especially cute, I discovered I had left the camera’s memory card in the card reader attached to my computer.
If you turn your camera on without a memory card installed, you’ll see a “No memory card” message at the top of the LCD. Furthermore, if you press the shutter button to take a picture, you’ll get another message (“Cannot record!”) on the LCD in combination with an audible signal—six chirps—to get your attention.
Cards are relatively inexpensive, so consider not only picking up one or two spares, but also storing them in a convenient place: a wallet, a purse, or a car’s glove compartment.