Here's a common error: practice v practise.
The former is a noun; the latter a verb.
(If you get stuck, think about advice and advise - because of the way you say these out loud [adv-ICE and adv-EYES], you never get them mixed up, do you?)
The lovely folk at I Bike Mcr need some practice. But we still love 'em.
Further to I Bike Mcr mention, go to this link for a vid from last week's Critical Mass. http://qik.com/video/1538744
Celia in front of mobile soundsystem; rider (me) has purple bag.