How will I convert a image into binary image?

How will I convert a image into binary image?

I = imread('D:\25.jpg');
>> BW = imbinarize(I)
??? Undefined function or method 'imbinarize' for input arguments of type 'uint8'.

Answer 1

[Kuifeng]

 doc im2bw

[abhishek nandi]

please give the full code

[Walter Roberson]

I = imread('D:\25.jpg');
BW = im2bw(I, 0.4);

[manoj kumar Yadav]

whta is the 0.4