1. Preheat oven to 250°F. Lightly oil a large sheet of foil on a baking sheet, then warm in oven.
2. Cook sugar and water in a deep 2-quart heavy saucepan over moderately low heat, stirring slowly with a fork, until melted and pale golden.
3. Cook caramel without stirring, swirling pan, until golden. If making the spicy variation, immediately stir in red pepper flakes, salt, and nuts and cook, stirring, until nuts are golden, about 1 minute.
4. Once brittle reaches hard-crack stage (about 300°F) quickly pour onto foil, tilting baking sheet to spread brittle evenly before it hardens. (If caramel hardens too fast, put in a 400°F oven until warm enough to spread, 1 to 2 minutes.) Cool brittle on baking sheet on a rack until completely hardened, then break into large pieces.
5. If you want to create brittle pops, cut skewers into thirds. Alternate the sticks on a foiled and oiled cookie sheet. Cook the brittle as directed above, pour on the foil covering 1/3 of the top of each stick. Once cooled, carefully break apart brittle pops.

Makes 1 1/2 cup brittle of each type.