It can seem like part of your brain quickly went offline. You might be grabbing milk from the fridge and suddenly your face feels funny. Or sitting at your desk and realize you can't budge your arm to answer the phone. Or in the middle of a sentence when you start slurring your words.
Any one of those three signs -- face drooping, arm weakness, and trouble talking -- means someone needs to call 911. Don't wait. And don't call your doctor or family members first.