As simple as striking a match. The top of the flare is covered by a cap. The cap has a cap on it, similar to the flip top of a shampoo bottle except it's not attached, that covered the striker. Really simple.

The flame is a very bright jet that fires straight out of the top. This means if you don't prop it up it shoots parallel to the ground.
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“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman