Spatula
A spatula is a flexible silicone or plastic blade on a handle, the primary tool for stirring and folding in pastry work. Its job is not to cut through a mixture but to gather and fold it while keeping the air in.
Flexibility decides everything. A stiff spatula does not follow the curve of the bowl and leaves unmixed pockets; an over-soft one cannot apply useful pressure. A good spatula bends under load and springs back.
The core technique is folding: draw the spatula down the side of the bowl, gather the mixture from the bottom and turn it over onto the surface, rotating the bowl as you go. This is how flour enters a sponge, meringue enters a mousse, whipped cream enters a custard base. Stirring in circles knocks the air out.
Silicone tolerates heat and works on the stove — the same tool stirs pastry cream and caramel. One-piece spatulas are preferable to assembled ones, since residue collects where handle meets blade.