Convert a non-deterministic Turing machine into a deterministic Turing machine
The deterministic machine simulates all possible computations of a nondeterministic machine, basically in parallel. Whenever there are two choices, the deterministic machine spawns two computations. This proces is sometimes called dovetailing. The tape of the deterministic simulator contains a list of configurations of the nondeterministic one, and performs a step on each of these in turn. This requires quite some administration, and the capability to move aroud data when one of the simulated configurations extends its allotted space.