So they're eating a crayon. Or a cricket. Or a dryer sheet. You get the picture. Something is in their mouth or hand and they need to drop it.
If you are normally rather calm and happy, letting out a good gasp and "No! Give it to Mommy," usually works. However, there are still the little guys who crave the attention and seeing Mommy get all worked up over nothing, so they let out a squeal of delight and take off with the offending object stuck in their oral cavity and inches away from their airway.
Please remember, you are (give or take) a good hundred pounds bigger than the critter in question, so you can outrun, overtake, and grab the source of angst with little or no problem. This may mean some screaming afterward, but it's the price we pay for keeping our duckling safe. The thing to watch out for is if this running away business is a bad habit. If so, it probably means a) the child is needing the attention, and positive attention or negative attention, it doesn't matter because he doesn't know the difference or b) you aren't really saying what you mean and doing what you say with any real consistency...and they know it.
I could give you some trite song and dance about trying to give them something else to distract them or negotiating a less offensive or destructive item for the one in question, but you aren't stupid and you know all this. Sure, we could all work on our presentation of making a cookie or string of beads seem much more attractive than a dead bird, but the bottom line is that they aren't stupid either. They know that you want what they have, and that means they have your undivided attention. So what this really boils down to is attention and consistency. Kids need positive attention, and if they aren't getting enough, they will go for negative attention. Period. Plus, the little guys will always, always try the running-away-with-a-mouthful-of-marbles thing at least a couple of times just to see what you will do. And if you always, always make sure they listen and do what you say, chances are, they won't try it often.
So make sure you are routinely giving them positive attention and guiding them on how to act, not just re-acting when they do something bad. And be consistent. Follow through on what you say day to day, hour to hour. It's a whip, but it shapes your child into a secure and happy little person.