Botanically speaking, vegetables do not exist. Show students a variety of what they might consider to be vegetables, such as carrots, beets, corn, tomatoes, green beans, radishes, cabbage, broccoli, cauliflower, and celery. Then ask them to determine what plant part each of these items represents in botanical terms--i.e., roots, seeds, fruit, leaves, flowers, or stems. They will soon understand that "vegetables" is a non-botanical, all-inclusive term.
Certain types of wildflowers, herbs, and portions of trees and shrubs are edible. Locate a guidebook for your local region and find a variety of edible plant parts (berries, flowers, and so forth) for demonstration in the classroom. Point out the hallmark traits that help identify each plant. Make an herbal tea from mint leaves (the smallest or youngest leaves taste best) and pass samples around to your students. Mint leaves work well since few people are allergic to this group of plants. Camomile, on the other hand, can trigger allergic responses in those that have problems with ragweed. Be sure students understand that some herbal remedies are tried and true, but with many others, caution must be used because claims can be exaggerated. Edible plant parts, on the other hand, are mostly nutritious counterparts of cultivated plants.
Emphasize that students must learn to properly identify plants before foraging on their own. Similar poisonous species must be recognized as well.