stretch *oneself / stand on tiptoe

straighten *one's back / straightening *one's back / stretch *oneself / standing on tiptoe / stand on tiptoe / stretching *oneself

push *oneself very hard to / overreach *oneself / aim too high

push *oneself very hard to / overreach *oneself / aim too high

push *oneself very hard to / overreach *oneself / aim too high

push *oneself very hard to / overreach *oneself / aim too high

setting *one's sights too high / set *one's sights too high / pushing $oneself very hard / push *oneself very hard to / overreaching *oneself / overreach *oneself / <<figurative>> bite off more than *one can chew / aiming too high / try to do *something beyond *one's power / aim too high

check the depth / sound [a river]

check the depth / sound [a river]

checking the depth (with *one's foot) / check the depth (with *one's foot)