Complex sentence: Though Alex could float by negating gravity, he had no means of self-propulsion, and thus could not truly fly; he initially depended on his sister Julie to push him along before developing an alternate method (see below).
Split into simpler sentences: Though Alex could float by negating gravity, he had no means of self-propulsion, and thus could not truly fly. He initially depended on his sister Julie to push him along before developing an alternate method (see below).