What to Braxton Hicks Contractions feel like?

Stacey
I'm 21 weeks pregnant with my second child. I never had a single contraction with my daughter. (Seriously!) I think I'm having Braxton Hicks Contractions now, but I'm not sure. What do they feel like? Does your belly harden from the outside like a real contraction or does your uterus feel tight (on the inside)?
Thanks!