Because of the enormous size of things in Brobdingnag, Gulliver has to be careful in his actions. During his first night in the Farmer's house, he trips on a crust of bread, and stares down the family cat. These do him no damage, but serve to impress on him the danger of his situation. The worst of his accidents come from the Farmer's children, who do not understand his fragility:
...his youngest son, who sat next to him, an arch boy of about ten years old, took me up by the legs, and held me so high in the air, that I trembled every limb...
...the child, who presently seized me by the middle, and got my head into his mouth, where I roared so loud that the urchin was frighted...
(Swift, Gulliver's Travels, eNotes eText)
However, he is not harmed by these experiences, only scared to be put at the mercy of the children. Later, Gulliver is attacked by a pair of rats, and only his skill in fencing protects him; the rats are as large as mastiff dogs. These incidents teach Gulliver that he needs to be very careful in dealing with the enormous world of Brobdingnag, or he will be suffer great harm.