Say you are in some godforsaken place that doesn't have a can of tahini on the shelf. (Yes, such places exist.) Here's how to make it yourself, from Jane Brody:
1/4 cup sesame seeds
1 teaspoon lemon juice
1/2 teaspoon oil
2 tablespoons water
Put everything in a blender and blend until smooth.
Note: You can grow your own sesame; you can see it in the garden at Monticello. It does need good drainage, which may be hard to come by in clay soils. You can order seed from Sesaco Corp. (800-527-1024)