The explanation of the mystery was really simple enough. The bewildered boys had been following a scientist who had just discovered how to make the human body transparent.

     Griffin, the scientist, had carried out experiment after experiment to prove that the human body could become invisible. Finally he swallowed certain rare drugs and his body became as transparent as a sheet of glass — though it also remained as solid as glass.
Explanation:
 
The narrator then says that the explanation of the mystery was quite simple and clear. It means that he was trying to let his readers know what was happening over there and whom the boys were following. The narrator then tells that the bewildered boys followed a scientist. He mentions them as "bewildered boys" because they were confused about the happenings. They were perplexed, and at the same time, they were eager to know about the invisible footprints. The narrator then says that the footprints belonged to a scientist who had discovered how to make the human body transparent or invisible.

The narrator then introduced the scientist Griffin to his readers. The scientist had worked for a long time and tested many experiments to see how to make the human body invisible. To know whether his discovery would become successful, he consumed certain rare drugs one day. As a result of consuming the rare drugs, the scientist became invisible as he expected. He became transparent like a sheet of glass. In addition, he says that the scientist seemed to be as solid as glass. It means that he had a definite shape and size but was invisible to others.
 
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Griffin, the scientist
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
Mystery Something strange or not known that has not yet been explained or understood
2
TransparentClear and easy to understand or recognize
3
Bewilder Confused and uncertain
4
InvisibleImpossible to see
5
ScientistAn expert who studies or works in one of the sciences
6
Swallow Cause or allow something, especially food or drink to pass down the throat
7
Rare Not common or frequent; very unusual
8
Discover To find information, a place, or an object, especially for the first time
9
DrugAny natural or artificially made chemical that is used as a medicine
10
Impression A mark made on the surface of something by pressing an object onto it
Reference:
National Council of Educational Research and Training (2007). Footprints without Feet - H.G. Wells(pp. 26-31). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.