make sure the lips of the lids are pointing down on the top and up on the bottom (to keep the tomatoes/grapes from rolling away) and use a large, sharp serrated knife. you may need practice to keep the knife level. restaurant workers use this method.
Cutting Grape Tomatoes (or grapes!) 1. Arrange grape tomatoes on their sides so that they're arrayed snugly in the top of the shallow plastic lid of a takeout container. 2. Place another, identical lid, upside down, over the tomatoes; hold the top lid down firmly. Using smooth, strong horizontal motions, pass the knife between the lids, slicing through the tomatoes as you go. 3. Remove the top lid and, voilà, your work is done.