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cutting Lithium battery strip/cut open put in water in can then wait for flame as hydrogen burns ..highly reactive

Mix a small amount of sodium chlorate( pulp mills have for bleaching paper) and sugar. Initiate the reaction by adding a few drops of sulfuric acid.( battery acid)

if you can get chlorine gas and steel wool…all you need is light… if in a dark wont work.(explosive if confined)

white phosphorus will auto ignite at 34 c ( find in munitions for smoke production (tanks)

if you run into an sr 71 blackbird the fuel auto ignites if over -20c (Triethylborane)

pure 120 mesh red phosphorus will ignite in strong sun light if laid out

there are other more exotic ways but those should only be done by skilled chemist