Thunks are values that have yet to be evaluated. For example '3 + 3' could be represented as a thunk, and there's no cost other than memory because you have not

Thunks are values that have yet to be evaluated. For example '3 + 3' could be represented as a thunk, and there's no cost other than memory because you have not computed '6'. So you're skipping the cost of computation for a value you may never use, but you still have to represent the expression in memory somewhere. This can lead to large thunks building up, eating up memory.