Samsung Galaxy S6 (SM-G920W8), Android 6.0.1
Find a dead bush in a desert biome or put some down in creative. Try to make it catch fire and burn up. It's a dead bush, it gives you sticks, it seems like it should catch fire just as easily as all the grasses and flowers.
Wiki Dead Bush: http://minecraft.gamepedia.com/Dead_Bush (identifies dead bush as flammable)
Wiki Flammables: http://minecraft.gamepedia.com/Fire#Burning_blocks (omits dead bush)
Flint and steel, lava, other things on fire beside dead bush... nothing makes it catch fire. It is dead. Dead even to fire. T-T