How did the invisible man first become visible?

Explanation

Griffin was completely invisible until he happened to step in some mud, which caused him to leave footprints as he walked. His footprints were seen by two boys, who followed him as long as the prints were visible. On getting rid of them, he went into a big London store to put on some warm clothes. After the store was shut, he got dressed by putting on shoes, an overcoat and a wide-brimmed hat. This made him visible to the people who came to work at the store the next day.