Lakes usually begin to freeze when the outside temperature starts dropping below 40 F but at 40 F the lake won't be totally frozen.
Then as the temperature starts dropping to 32 F the lake begins to freeze even more and harder and the lower the temperature the more the lake begins freezing.
Still even though it's 32 F out and the lake seems frozen solid it's still really not safe to go onto the frozen lake.
For a lake to be fully frozen the temperature outside should be between 10 F or below and the cold it is the more the lake freezes.
So be very careful if you do decide to go Ice Skating on a frozen pond or lake because not always is the lake or pond frozen over enough to skate on or walk on.
People have fallen into lakes and ponds and died even though it was freezing out but the pond or lake was not thick enough frozen to allow the weight to be on it.