It's and Its.
These two cause all sorts of problems and it is well worth the two minutes it takes to understand the difference between the two.
It's is the contracted form of 'it has' or 'it is' and is used in the following ways:
"It's been a long time since we spoke," he whispered. (it has)
"Come on," he shouted, "it's a lovely day!" (it is)
"There is no way it's going to be ready on time."(it is)
"It's been ready for weeks!" (it has)
Its is the possessive form of it, meaning 'of it'. This is possibly why the difference between it's and its causes so many problems. Its, without an apostrophe, is a possessive form, where an apostrophe is usually required. It is similar to words like his and her, neither of which needs an apostrophe.
The building was missing its doors and windows.
The tree had lost all of its leaves.
Has your chewing gum lost its flavour?
Madrid is famous for its art galleries.
Test your understanding of it's and its with this exercise.