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turn on physics /os map physics 0-5

or /physics 0-5

0 - off (tnt will not explode)

1 - normal (tnt will not explode like you want to explode it)

2 - advance (tnt will explode when you place fire next to it) USE THIS ONE 

3 - hardcore (tnt will explode when you place fire next to it)

4 - instant (tnt will explode when you place fire next to it)

5 - doors olny (tnt will not explode, olny will doors work)

if u are trying to make tnt explode in ur own realm u could just type /mode tnt <size> the sizes are small big and nuke. now these have a 5 second count down before they explode after u place it (it will kill u as well) now the tnt in this case wont go off if u set it on fire it goes of 5 seconds after u placed it.