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Sometimes it can be tricky to know if tickling will be viewed as cute and playful or annoying and inappropriate. Consider your location (if you are in public, he might not appreciate giggling madly in front of a bunch of people), and his current mood. If you've just been fighting or he's angry, now might not be a good time to poke and prod him. Listen to what he's saying. If he's confiding in you that he's heartbroken over his dog's death, he probably needs a hug more than he needs tickle torture. Wait until you're somewhere relatively private and have room to wrestle a little and not injure yourselves. Make sure he's not holding a hot beverage or something that might injure him or you when you catch him by surprise. Don't try to tickle him when he's playing video games or absorbed in something that is really important to him (even if you are annoyed that he's not paying attention to you). This is when tickling is just annoying, not fun. If you two are joking around and he's in a good mood, he probably won't mind a little tickle. Your tickle attack will fail if it turns out your boyfriend isn't even ticklish. Do a little exploring first to figure out where he's most ticklish. The most vulnerable spots on the body are also the most ticklish Poke him in his side, just above his hip or in his ribs and see if he giggles. Give him a hug and "accidentally" tickle his armpit. This is one of the more ticklish spots on most people. If he's laying down with his shoes off, gently run your toes along the bottom of his feet and notice his reaction. Kiss him gently on the neck and exhale through your nose so the air hits his ear. See if he gets goosebumps on his neck. Sharp nails might be good for tickling if you plan to just graze them over his skin (like on the underside of his feet or the back of his neck). But if you plan to really get into a tickle war, sharp nails may become painful.
Determine if it's the right time and place. Figure out if he's ticklish. Make sure your nails aren't too sharp.