The most common types of physical bitcoins are Casascius coins, Titan coins, and Denarium coins. Casascius coins are metal coins with a private key hidden inside a hologram. Titan coins are made of titanium and have a private key engraved on the back. Denarium coins are plastic cards with a private key hidden under a scratch-off sticker. To use a physical bitcoin, the private key must be accessed and transferred to a digital wallet.