A paddle is a sex toy with a handle and two flat sides that's usually made of leather or wood. Paddles are often used as part of BDSM or bondage play to give the receiver a light or heavy spank on the bottom. In BDSM, pain is used as a way to enhance sensation and pleasure.
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There are lots of great, unintimidating paddles out there if you're curious about spanking. One key thing to remember when paddling or spanking someone is to take care about where you hit. The butt, thighs, and even the upper back (gently) can be safely spanked. Additionally, so can the breasts (again, gently). However, striking near the kidneys, on the lower back, neck, face, or joints should be avoided, as this is more likely to cause injury.